Partially reflecting multilayer optical films with reduced color

ABSTRACT

A multilayer optical film body includes a first and second packet of microlayers. Each packet partially transmits and partially reflects light over an extended wavelength range, such as the visible region, for normally incident light polarized along a first principal axis of the film body. In combination, the first and second packets have an intermediate reflection and transmission (e.g. 5-95% internal transmission, on average) for the normally incident light, and similar intermediate reflection/transmission (e.g. 10-90% internal transmission, on average) for oblique light. The packets are laminated or otherwise connected so that light can pass through the packets sequentially. In at least a first test area of the film body, a high frequency spectral variability of the combination of packets is less than a high frequency spectral variability of the first packet by itself, and may also be less than a high frequency spectral variability of the second packet by itself.

FIELD OF THE INVENTION

This invention relates generally to optical films whose reflection andtransmission characteristics are determined in large part byconstructive and destructive interference of light reflected frominterfaces between microlayers within the film, with particularapplication to such films that partially reflect and partially transmitlight over an extended wavelength range for a given incidence condition.The invention also relates to associated articles, systems, and methods.

BACKGROUND

Multilayer optical films are known. Such films typically incorporate alarge number of very thin layers of different light transmissivematerials, the layers being referred to as microlayers because they arethin enough so that the reflection and transmission characteristics ofthe optical film are determined in large part by constructive anddestructive interference of light reflected from the layer interfaces.Depending on the amount of birefringence (if any) exhibited by theindividual microlayers, and the relative refractive index differencesfor adjacent microlayers, and also on other design characteristics, themultilayer optical films can be made to have reflection and transmissionproperties that may be characterized as a reflective polarizer in somecases, and as a mirror in other cases, for example.

Reflective polarizers composed of a plurality of microlayers whosein-plane refractive indices are selected to provide a substantialrefractive index mismatch between adjacent microlayers along an in-planeblock axis and a substantial refractive index match between adjacentmicrolayers along an in-plane pass axis, with a sufficient number oflayers to ensure high reflectivity for normally incident light polarizedalong one principal direction, referred to as the block axis, whilemaintaining low reflectivity and high transmission for normally incidentlight polarized along an orthogonal principal direction, referred to asthe pass axis, have been known for some time. See, e.g., U.S. Pat. No.3,610,729 (Rogers), U.S. Pat. No. 4,446,305 (Rogers et al.), and U.S.Pat. No. 5,486,949 (Schrenk et al.).

More recently, researchers from 3M Company have pointed out thesignificance of layer-to-layer refractive index characteristics of suchfilms along the direction perpendicular to the film, i.e. the z-axis,and shown how these characteristics play an important role in thereflectivity and transmission of the films at oblique angles ofincidence. See, e.g., U.S. Pat. No. 5,882,774 (Jonza et al.). Jonza etal. teach, among other things, how a z-axis mismatch in refractive indexbetween adjacent microlayers, more briefly termed the z-index mismatchor Δnz, can be tailored to allow the construction of multilayer stacksfor which the Brewster angle—the angle at which reflectance ofp-polarized light at an interface goes to zero—is very large or isnonexistent. This in turn allows for the construction of multilayermirrors and polarizers whose interfacial reflectivity for p-polarizedlight decreases slowly with increasing angle of incidence, or isindependent of angle of incidence, or increases with angle of incidenceaway from the normal direction. As a result, multilayer films havinghigh reflectivity for both s- and p-polarized light for any incidentdirection in the case of mirrors, and for the selected direction in thecase of polarizers, over a wide bandwidth, can be achieved.

Some multilayer optical films are designed for narrow band operation,i.e., over a narrow range of wavelengths, while others are designed foruse over a broad wavelength range such as substantially the entirevisible or photopic spectrum, or the visible or photopic wavelengthrange together with near infrared wavelengths, for example. Over theyears, designers and manufacturers of the latter type of films, i.e.,broadband multilayer optical films, have had to deal with the issue ofcolor. The color issue often arises when the film is intended for use ina visual display system, e.g., where the film is a broadband reflectivepolarizer or a broadband mirror, and the display system is a liquidcrystal display, luminaire, or backlight. In such systems, it istypically undesirable for the film to impart a significant colored(non-white) appearance to the display, whether at normal incidence orfor obliquely incident light. The colored appearance occurs when thefilm has transmission or reflection characteristics that are not uniformover the visible portion of the spectrum. In the case of coextrudedpolymeric multilayer optical films, such non-uniformities are typicallythe result of imperfect control of the layer thickness profile of thefilm relative to a target profile. To avoid the color issue, polymericmultilayer optical films are often designed to provide along theirprincipal axes either extremely low reflectivity and high transmission(e.g. for a pass axis of a reflective polarizer) or extremely highreflectivity and low transmission (e.g. for a block axis of a reflectivepolarizer, or for any in-plane axis of a reflective mirror film).Forcing the reflectivity to extremely low or extremely high values (andthe transmission to extremely high or extremely low values,respectively) results in low-color broadband films, because themagnitude of spectral non-uniformities in reflection or transmission issmaller for a given variability in layer thickness profile when thenominal reflectivity is near 0 (0%) or 1 (100%), and the nominaltransmission is near 1 or 0, respectively.

Recently, broadband polymeric multilayer optical films have beenproposed that have intermediate amounts of reflectivity and transmissionfor light polarized parallel to at least one principal optic axis sothat some significant amount of incident light is reflected, and anothersignificant amount of the incident light (typically, the remainder ofthe incident light that is not reflected) is transmitted. Such films arereferred to herein as partially reflecting multilayer optical films, orpartially transmitting multilayer optical films. One approach toaddressing color issues in such films is to provide them with only asingle packet of microlayers with a carefully tailored layer thicknessprofile, and to manufacture them without the use of any layer multiplierdevices, to provide maximum control of the layer thickness profile and acorresponding minimum spectral variability in transmission or reflectionover the visible wavelength range. Reference is made, for example, toPCT publication WO 2009/0123928 (Derks et al.), “Low Layer CountReflective Polarizer With Optimized Gain”.

BRIEF SUMMARY

Thus, as already mentioned above, one challenge faced by designers andmanufacturers of polymeric multilayer optical films that are intended tobe both (1) partially reflecting along a principal axis and (2)broadband (i.e., intended to provide partial reflectivity over a broadwavelength range) is unintended and undesired perceived color resultingfrom imperfect layer thickness control. Such undesired color istypically manifested as relatively high frequency variability in theoptical transmission and reflection spectra, the high frequencyvariability being directly associated with deviations in the thicknessesof the microlayers (stated more accurately, deviations in the opticalthicknesses of optical repeat units of microlayers) from their ideal ortarget values. This challenge facing manufacturers and designers ofpartially reflecting broadband multilayer optical films is explainedfurther below with reference to FIGS. 1-3, in the Detailed Descriptionportion of this document.

Multilayer optical films of particular interest to the presentapplication are films that are partially reflective and partiallytransmissive over a wide wavelength range of interest, along at leastone principal in-plane axis, both for normally incident light andobliquely incident light. Such films may be characterized by the“internal transmission” of the film or of one or more of its constituentcomponents for a specified light incidence condition. The “internaltransmission” of a film refers to the transmission of the film when anyeffects of the front-most and rear-most surfaces of the film/component,which surfaces may or may not be in contact with air, are not includedin the measurement.

Thus, the internal transmission of a film refers to the transmissionthat results only from interior portions of the film/component, and notthe two outer surfaces thereof. Completely analogous to internaltransmission is “internal reflection”. Thus, the internal reflection ofa film refers to the reflection that results only from interior portionsof the film/component, and not the two outermost surfaces thereof.Characterizing a multilayer optical film or other optical body in termsof its internal transmission and/or internal reflection can bebeneficial and helpful in many situations. For clarity, when terms suchas “reflection” and “transmission” (and related terms such asreflectivity and transmissivity) are used in this document in connectionwith a multilayer optical film or other optical body or portion thereof,the reader will understand that they may refer, depending on thecontext, to the ordinary reflection and transmission of the body, whichusually (except for p-polarized light incident at Brewster's angle)include the effects of the two outermost surfaces, or to the internalreflection and transmission of the body, or to both. When the effects ofthe outermost surfaces are intended to be specifically excluded, theadjective “internal” is used throughout this document.

The present application discloses, among other things, partiallytransmitting multilayer optical film bodies that result in reduced colorby combining, for example by lamination, two separate multilayer opticalfilms or microlayer packets whose respective high frequency spectralpeaks and valleys tend to cancel each other over a broad wavelengthrange of interest and at at least some areas of the film body. Inpractice, multilayer optical films and microlayer packets can exhibitspatial non-uniformities such that spectral peaks and valleys shiftslightly in wavelength from one area or place on the film or packet tothe next. Even when such spatial non-uniformities result in another areaof the construction at which the respective spectral peaks and valleysof the different films or packets do not tend to cancel each other, wehave found that the color of such non-optimal areas typically increasesby only a modest amount, such that the overall construction may stillhave a reduced color on average, where the average may include a spatialaverage over the useful area of the film construction.

Thus, for example, partially reflective coextruded multilayer opticalfilm bodies can be made that have, compared to previous designs, aflatter or smoother transmission spectrum over the wavelength range ofinterest, at least in some areas or portions of the film body, and/orwhen considering a spatial average over all points on the film body. Thefilm bodies may be made with two or more multilayer packets and two ormore corresponding continuous layer profiles, slightly adjusted inthickness or shape with respect to one another, where the reflectionband for each packet substantially spans the entire wavelength range ofinterest. This is different from film bodies that use only onemultilayer packet and only one corresponding continuous layer profilewhose reflection band spans the wavelength range of interest. Such asingle packet film body can be considered to be more efficient than thedescribed film bodies made with two or more multilayer packets, becausethe single packet film body can use a smaller total number ofmicrolayers to produce the reflection band with a specified amount ofreflectivity over the wavelength range of interest. See e.g. PCTpublication WO 2009/0123928 (Derks et al.). But even though thedisclosed multi-packet designs are typically less efficient from anoptical standpoint than single packet designs, we have found that theycan provide better overall spectral uniformity across the wavelengthrange when considering the entire useable area of the film body.

The present application therefore discloses, among other things,multilayer optical film bodies that include a first and second packet ofmicrolayers. In some cases, the film body may include no microlayerpackets other than the first and second packets, while in other casesthe film body may include one or more other microlayer packets. Thefirst packet of microlayers may be configured to partially transmit andpartially reflect light over an extended wavelength range, such as thevisible region, for normally incident light polarized along a firstprincipal axis of the film body, and the second packet of microlayersmay also be configured to partially transmit and partially reflect lightover the extended wavelength range for the same normally incidentlinearly polarized light. In combination, the first and second packetsmay have a significant amount of reflection and transmission (e.g.,internal transmission, averaged over the extended wavelength range, in arange from 5-95%) for the normally incident light, and similar partialreflection and transmission for oblique light, e.g., p-polarized lightincident from air at a 60 degree angle in a plane containing the firstprincipal axis. In combination, the first and second packets may have aninternal transmission in a range from 0.1 (10%) to 0.9 (90%), or from0.2 to 0.8, or from 0.3 to 0.7, for the 60 degree p-polarized light whenaveraged over the extended wavelength range. In some cases, the internaltransmission for the 60 degree oblique p-polarized light may be lessthan the internal transmission for the normally incident light, e.g., ifthe microlayers reflect p-polarized light more strongly with increasingincidence angle. In other cases, the internal transmission for the 60degree oblique p-polarized light may be greater than the internaltransmission for the normally incident light, e.g., if the microlayersreflect p-polarized light more weakly with increasing incidence angle.In still other cases, the internal transmission for the 60 degreeoblique p-polarized light may be substantially the same as the internaltransmission for the normally incident light.

The first and second packets of microlayers may be connected such thatat least some light can pass through the first and second packets ofmicrolayers sequentially. In at least a first test area of themultilayer optical film body, a high frequency spectral variability(Δcomb) of the combination of first and second packets may be less thana high frequency spectral variability (Δ1) of the first packet byitself, and may also be less than a high frequency spectral variability(Δ2) of the second packet by itself. In some cases, Δcomb may be thesame as the high frequency spectral variability ΔFB of the multilayeroptical film body over the same extended wavelength range, particularlywhen the film body includes no microlayer packets other than the firstand second packet.

The first packet of microlayers may exhibit a first transmissionspectrum over the extended wavelength range for the normally incidentlight, the first transmission spectrum having the first high frequencyspectral variability Δ1. The second packet of microlayers may exhibit asecond transmission spectrum over the extended wavelength range for thenormally incident light, the second spectrum having the second highfrequency spectral variability Δ2. A difference between the first andsecond transmission spectra may yield a first differential transmissionspectrum over the extended wavelength range, the first differentialtransmission spectrum having a first differential high frequencyspectral variability Δdiff. A combination of the first and secondtransmission spectra may yield a first combination transmission spectrumover the extended wavelength range, the first combination transmissionspectrum having the high frequency spectral variability Δcomb. Themultilayer optical film body may exhibit a first film body transmissionspectrum over the extended wavelength range for the normally incidentlight, the first film body transmission spectrum having the first filmbody high frequency spectral variability ΔFB. (In cases where themultilayer optical film body consists essentially of the first andsecond packets of microlayers, with at least one or more optically thicklight transmissive layers, the first combination transmission spectrummay be substantially the same as the first film body transmissionspectrum, and Δcomb may be substantially the same as ΔFB.) The film bodyis preferably constructed in such a way that Δdiff is greater than atleast one of Δ1 and Δ2. Furthermore: Δdiff may be greater than each ofΔ1 and Δ2; Δcomb and/or ΔFB may be less than at least one of Δ1 and Δ2;and Δcomb and/or ΔFB may be less than each of Δ1 and Δ2.

The first transmission spectrum, the second transmission spectrum, thefirst differential transmission spectrum, the first combinationtransmission spectrum, and the first film body spectrum may be internaltransmission spectra, or they may be ordinary (external) transmissionspectra. These transmission spectra may each have a measurementresolution of 5 nm or less, which is well within the capability of mostspectrophotometers. The first test area may be selected such that anygiven spectral feature of the film body shifts in wavelength by lessthan a specified amount, e.g. about 5 nm, between any two portions ofthe first test area. For many multilayer optical film bodies and mostspectrophotometers, this test area ranges from about 1 mm² to 1 cm². Theextended wavelength range of interest may include at least a majority ofa range from 400 nm to 700 nm, e.g., from 420 to 680 nm, or from 420 nmto a wavelength greater than 680 nm, such as 420 to 1000 nm.

Any given high frequency spectral variability may be a variability basedon a difference between the pertinent transmission spectrum and abest-fit curve to the transmission spectrum over the wavelength range ofinterest, the best-fit curve being of the form a₀+a₁λ, +a₂λ²+a₃λ³, forexample, where λ is the optical wavelength. The high frequency spectralvariability may be a standard deviation of the difference between thetransmission spectrum and the corresponding best-fit curve. Thus, Δ1 maybe based on a difference between the first transmission spectrum and afirst best-fit curve to the first transmission spectrum over thewavelength range of interest, the first best-fit curve being of the forma₀+a₁λ, +a₂λ²+a₃λ³. The parameter Δ2 may similarly be based on adifference between the second transmission spectrum and a secondbest-fit curve to the second transmission spectrum over the wavelengthrange of interest, the second best-fit curve also being of the forma₀+a₁λ, +a₂λ²+a₃λ³. The parameter Δdiff may also be based on adifference between the first differential transmission spectrum and afirst differential best-fit curve to the first differential transmissionspectrum over the wavelength range of interest, the first differentialbest-fit curve also being of the form a₀+a₁λ, +a₂λ²+a₃λ³. The first andsecond packets in combination may exhibit a first combinationtransmission spectrum over the extended wavelength range for thenormally incident light, the first combination transmission spectrumhaving the high frequency spectral variability Δcomb, where Δcomb may bebased on a difference between the first combination transmissionspectrum and a first combination best-fit curve to the first combinationtransmission spectrum over the wavelength range of interest, the firstcombination best-fit curve also being of the form a₀+a₁λ, +a₂λ²+a₃λ³.

The first and second packets may be the same or similar to each other inconstruction, and may have the same or similar transmission andreflection characteristics when considered individually. The firsttransmission spectrum, for example, may have a first average value overthe wavelength range of interest, and the second transmission spectrummay have a second average value over the same wavelength range, and thefirst average value may differ from the second average value by lessthan 0.2, or less than 0.1, for example. The first average value may besubstantially the same as the second average value.

The first packet may have substantially the same number of microlayersas the second packet. The first and second packets of microlayers mayeach be characterized by nominally monotonic layer thickness profiles ofoptical repeat units.

In a second test area of the multilayer optical film body different fromthe first test area, the first packet may exhibit a third transmissionspectrum over the extended wavelength range for the normally incidentlight, the third transmission spectrum having a third high frequencyspectral variability Δ3, and the second packet of microlayers mayexhibit a fourth transmission spectrum over the extended wavelengthrange for the given incidence condition, the fourth transmissionspectrum having a fourth high frequency spectral variability Δ4. Adifference between the third and fourth transmission spectra may yield asecond differential spectrum over the extended wavelength range, thesecond differential spectrum having a second differential high frequencyvariability Δdiff2. In the second test area, the combination of thefirst and second packets may exhibit a second combination transmissionspectrum over the extended wavelength range for the normally incidentlight, the second combination transmission spectrum having a secondcombination high frequency spectral variability Δcomb2. Δdiff2 may insome cases be less than at least one of Δ3 and Δ4. Furthermore: Δdiff2may be less than each of Δ3 and Δ4; and Δcomb2 may be greater than atleast one of Δ3 and Δ4.

Also disclosed are methods of making partially reflective multilayeroptical film bodies, such methods including providing a first packet ofmicrolayers, providing a second packet of microlayers, and connectingthe first and second packets to form the multilayer optical film bodysuch that at least some light can pass through the first and secondpackets sequentially. The first packet may be configured to partiallytransmit and partially reflect light over an extended wavelength rangefor normally incident light polarized along a first principal in-planeaxis, and the second packet may also be configured to partially transmitand partially reflect light over the extended wavelength range for thenormally incident light. The connecting may be carried out such that, atleast in a first test area, the first packet exhibits a firsttransmission spectrum over the extended wavelength range for thenormally incident light, the first transmission spectrum having a firsthigh frequency variability Δ1, and the second packet exhibits a secondtransmission spectrum over the extended wavelength range for thenormally incident light, the second transmission spectrum having asecond high frequency variability Δ2. The combination of the first andsecond packets may have a first combination transmission spectrum overthe extended wavelength range for the given incidence condition, thefirst combination transmission spectrum having a first combination highfrequency variability Δcomb, where Δcomb may be less than at least oneof Δ1 and Δ2, and is preferably less than each of Δ1 and Δ2.

Also disclosed are methods of making partially reflective multilayeroptical film bodies, such methods including providing a first packet ofmicrolayers, providing a second packet of microlayers, and connectingthe first packet to the second packet to form the multilayer opticalfilm body, where at least some light can pass through the first andsecond packets sequentially. The first packet may be configured topartially transmit and partially reflect light over an extendedwavelength range for a given incidence condition, and the second packetmay also be configured to partially transmit and partially reflect lightover the extended wavelength range for the given incidence condition.The connecting may be carried out such that, at least in a first testarea of the multilayer optical film body, a high frequency spectralvariability of the combination of first and second packets, and/or ofthe film body, is less than a high frequency spectral variability of thefirst packet of microlayers, and preferably also less than a highfrequency spectral variability of the second packet of microlayers.

Also disclosed are partially reflective multilayer optical film bodiesthat include a first and second packet of microlayers. The first packetof microlayers may be configured to partially transmit and partiallyreflect light over an extended wavelength range for a given incidencecondition, and the second packet of microlayers may also be configuredto partially transmit and partially reflect light over the extendedwavelength range for the given incidence condition, the first and secondpackets being connected so that at least some light can pass through thefirst and second packets of microlayers sequentially. At least in afirst test area of the multilayer optical film body, a high frequencyspectral variability of the combination of first and second packets,and/or of the film body, may be less than a high frequency spectralvariability of the first packet of microlayers, and preferably less thana high frequency spectral variability of the second packet ofmicrolayers.

Related methods, systems, and articles are also discussed.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side view of an idealized single packet partiallyreflective broadband multilayer optical film;

FIG. 2 is a schematic side view of a multilayer optical film similar tothat of FIG. 1, but where undesired variability in the optical thicknessof the optical repeat units produces undesired high frequencyvariability in the reflection and transmission spectra for the film;

FIG. 3 is a schematic side view of a multilayer optical film similar tothat of FIG. 2, but where the microlayers in the film are divided intotwo distinct packets;

FIG. 4 is a schematic perspective view of an exemplary optical repeatunit (ORU) of a multilayer optical film;

FIG. 5 is a schematic perspective view of a portion of a multilayeroptical film, this view showing a packet of microlayers and a pluralityof ORUs;

FIG. 6 is a graph of a portion of a measured thickness profile of anactual multilayer optical film;

FIG. 7a is a plot of spectral reflectivity for two hypotheticalmicrolayer packets, the figure also showing a difference spectrumbetween the two microlayer spectral reflectivities, and a combinationspectrum that results when the two hypothetical packets are incorporatedinto a hypothetical 2-packet multilayer optical film body;

FIGS. 7b and 7c are plots similar to FIG. 7a , but where the spectralreflectivity of one of the microlayer packets is shifted in wavelength;

FIG. 8 is a perspective view of a multilayer optical film body, showingexemplary test areas;

FIG. 9a is a graph of measured spectral transmission for a 2-packetmultilayer optical film body, and for its two constituent microlayerpackets individually after they were physically separated from eachother;

FIG. 9b is a graph showing the measured spectral transmission of the2-packet film body of FIG. 9a , together with a graph of calculatedspectral transmission based on the measured spectral transmission forthe two constituent microlayer packets of FIG. 9 a;

FIG. 10 is a graph showing the measured spectral transmission of another2-packet multilayer optical film, and of its two constituent microlayerpackets individually after they were physically separated from eachother;

FIG. 11 is a graph showing the measured spectral transmission of twosingle-packet multilayer optical film bodies that were subsequentlylaminated together to form a 2-packet multilayer optical film body; and

FIGS. 12a-d are additional graphs that are based on the measuredspectral transmission of two single-packet multilayer optical filmbodies, where some manipulation of the data was performed to simulatethe fabrication of several different 2-packet multilayer optical filmbodies.

In the figures, like reference numerals designate like elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

As mentioned above, one challenge faced by designers and manufacturersof polymeric multilayer optical films that are intended to be both (1)partially reflecting along a principal axis at normal and oblique anglesand (2) broadband (i.e., intended to provide partial reflectivity over abroad wavelength range) is unintended and undesired color resulting fromimperfect layer thickness control. Such undesired color is typicallymanifested as relatively high frequency variability in the opticaltransmission and reflection spectra, the high frequency variabilitybeing directly associated with deviations in the thicknesses of themicrolayers (stated more accurately, deviations in the opticalthicknesses of optical repeat units of microlayers) from their ideal ortarget values. This challenge facing manufacturers and designers ofpartially reflecting broadband multilayer optical films will now beexplained with reference to FIGS. 1-3. For purposes of these figures,for simplicity, the multilayer optical film bodies are assumed to haveno spatial variability in the plane of the film body. Thus, the spectralreflection and transmission characteristics of a given film body areassumed to be independent of the position or location on the film (e.g.,the (x,y) coordinate) at which they are measured.

In FIG. 1, a multilayer optical film body 110 is illuminated by light112 having a particular incidence condition. For example, the light maybe incident along a direction that is normal (perpendicular) to theplane of the film body, and the light may be unpolarized. Alternatively,the light may be incident along a different direction, and the light mayhave a different polarization characteristic, e.g., linearly polarizedalong a particular direction, or circularly polarized, or polarized insome other way. The reader will understand that the particular incidencecondition, which may be characterized by both a direction of incidenceand a polarization state, may be specified as desired by the opticalfilm designer or manufacturer. The film body 110 is shown in the contextof a Cartesian x-y-z coordinate system, and is drawn as being flat,extending parallel to the x-y plane. In general, the film body need notbe flat, and if not then any sufficiently small portion of the film bodymay be considered to be flat in isolation. The body 110 is referred toas a “multilayer optical film body” because it is a body that includes amultilayer optical film. In some cases, the multilayer optical film maybe the only component of the film body, i.e., the film body and themultilayer optical film may be identically the same. In other cases, thefilm body may include other components, e.g., one or more substrates,layers, coatings, films (including one or more additional multilayeroptical films), or the like, to which the multilayer optical film islaminated, adhered, or otherwise attached.

Still referring to FIG. 1, the film body 110 has only one contiguousstack or packet of microlayers arranged into optical repeat units(ORUs), which are discussed further below and shown only schematicallyin FIG. 1. The ORUs have optical thicknesses (physical thicknessmultiplied by refractive index, the thickness measured along thez-direction) that change from one end of the packet to the other, suchthat a plot 114 of optical thickness versus the ORU number, counted fromone side of the packet to the other, produces a monotonically varyingthickness profile, although other desired functional shapes may also beused for the thickness profile. The thickness profile of plot 114 isassumed to be ideal, with no deviations in the optical thickness of theORUs from their target values. Furthermore, the thickness profile, andthe optical properties of the various microlayers in the packet, and thetotal number of microlayers (and the total number of ORUs) are assumedto be selected to provide an intermediate amount of reflectivity and anintermediate amount of transmission, for example, a reflectivity ofabout 0.50 (50%), over a broad wavelength range from λ₁ to λ₂, as can beascertained from the power spectra shown in plots 116, 118, 120. Plot116 shows power per unit wavelength as a function of wavelength for theincident light 112, indicating the incident light 112 is from anidealized broadband emitter, e.g., light from a white light source. Plot118 shows power per unit wavelength as a function of wavelength for theportion of the incident light 112 that is reflected by the film body110. Plot 120 shows power per unit wavelength as a function ofwavelength for the portion of the incident light 112 that is transmittedby the film body 110.

The spectral reflection and transmission characteristics of the filmbody 110 can be readily ascertained by dividing the function in plot 118by the function in plot 116, and by dividing the function in plot 120 bythe function in plot 116, respectively. Inspection of FIG. 1 shows thatsuch division operations will yield reflection and transmission spectrafor the idealized film body 110 that are substantially constant betweenthe limits of λ₁ and λ₂, with a constant value (and average value) ofabout 0.5, or 50%, for each parameter and no high frequency deviationsor variability from that value as a function of wavelength.

Turning now to FIG. 2, we see there a single packet multilayer opticalfilm body 210 similar to film body 110, but where the opticalthicknesses of the ORUs in the film body 210 are shown to includedeviations from their ideal or target values. Thickness deviations ofmicrolayers or ORUs from their target values can be reduced or minimizedthrough careful design and control of the film manufacturing process,but in practical embodiments some non-trivial thickness deviations willremain. (See, for example, the measured thickness gradient of FIG. 6,which is discussed further below.) Broadband light 112 again impinges onthe film body at the particular incidence condition, and some light isreflected by the film body, while some is transmitted.

Similar to film body 110, film body 210 has a single contiguous stack orpacket of microlayers arranged into ORUs, which are shown onlyschematically in FIG. 2. The ORUs have optical thicknesses that changefrom one end of the packet to the other as shown in plot 214, whichplots optical thickness against the ORU number, producing a generallymonotonic thickness profile. The thickness profile of plot 214 has thesame general shape as that of plot 114 (FIG. 1), but the thicknessprofile of plot 214 is non-ideal, having significant deviations in theoptical thickness of the ORUs from their target values. Such deviationsmay arise from a variety of factors during the manufacture of themultilayer optical film. The thickness profile, and the other relevantcharacteristics of the packet, are assumed to be selected to provide anintermediate amount of reflectivity and an intermediate amount oftransmission, for example, a reflectivity of about 0.50 (50%), over abroad wavelength range from λ₁ to λ₂, as can be ascertained from thepower spectra plotted in plots 116, 218, 220. Since like referencenumerals designate like elements, plot 116 in FIG. 2 again indicates theincident light 112 is from an idealized broadband emitter, e.g., anideal white light source. Plot 218 shows power per unit wavelength as afunction of wavelength for the portion of the incident light 112 that isreflected by the film body 210. Plot 220 shows power per unit wavelengthas a function of wavelength for the portion of the incident light 112that is transmitted by the film body 210.

The spectral reflection and transmission characteristics of the filmbody 210 can be ascertained by dividing the function in plot 218 by thefunction in plot 116, and by dividing the function in plot 220 by thefunction in plot 116, respectively. Such division operations will yieldreflection and transmission spectra for the non-idealized film body 210that, between the limits of λ₁ and λ₂, have average values of about 0.5,or 50%, for each parameter, but that have significant high frequencydeviations from that value as a function of wavelength. The variationsin the reflection and transmission spectra, and in the plots ofreflected and transmitted light 218, 220, are the direct result of thevariations in the thickness profile of the ORUs shown in plot 214.

Variations in reflection and transmission spectra such as are depictedin plots 218, 220 may or may not be acceptable to the system designer,depending on various factors such as the amplitude of the variations,the type of light source used, the type of detector used, and whetherother optical components (such as a diffuser) are present in the opticalsystem. In at least some cases, the spectral variations may introduce anunacceptable amount of perceived color into the system.

We have developed techniques for producing multilayer optical filmbodies that have a reduced amount of high frequency spectral variationin the reflection and transmission spectra, for a given variability inthe thickness profile of the ORUs. These techniques generally involvearranging the microlayers into distinct multiple packets of microlayers(e.g., a first and second packet of microlayers) separated by at leastone optically thick layer, where each packet of microlayers has areflection band spanning the entire bandwidth of the intended reflectordesign, and then judiciously combining the multiple packets in such away as to reduce the spectral variability. These techniques aredescribed in more detail below. First, however, we refer to theschematic view of FIG. 3 to illustrate the technique in a generalizedway.

Unlike film bodies 110 and 210, film body 310 of FIG. 3 has microlayersthat are organized into two distinct stacks or packets of microlayersarranged into ORUs. The packets are shown schematically in FIG. 3 aspackets 310 a and 310 b, and are separated by an optically thick layer310 c. The ORUs have optical thicknesses that change from one end of thefilm body to the other as shown in plot 314. Two distinct monotonicthickness profiles can be seen, corresponding to the distinct microlayerpackets 310 a, 310 b. As shown, each of the ORU thickness profiles isnon-ideal, having significant deviations in the optical thickness of theORUs from their target values. Further, the different ORU thicknessprofiles substantially overlap each other in their optical thicknessdistributions: the minimum ORU optical thickness of the differentprofiles are the same or similar, and the maximum ORU optical thicknessof the different profiles are also the same or similar. This causes thedifferent packets 310 a, 310 b to have individual reflection bands thatsubstantially overlap each other, e.g., having the same or similarminimum wavelength Xi, and the same or similar maximum wavelength λ₂.The reflection band of one packet may, for example, overlap at least70%, 80%, or 90% or more of the reflection band of the other packet. Thecombination (but not simply the arithmetic sum) of the two individualreflection bands produces the reflection band of the film body 310.

The thickness profile, and the other relevant characteristics of thepacket, are assumed to be selected to provide the film body 310 with anintermediate amount of reflectivity and an intermediate amount oftransmission, for example, a reflectivity and transmission of about 0.50(50%), over a broad wavelength range from λ₁ to λ₂, as can beascertained from the power spectra plotted in plots 116, 218, 220. Sincelike reference numerals designate like elements, plot 116 in FIG. 2again indicates the incident light 112 is from an idealized broadbandemitter, e.g., an ideal white light source.

FIG. 3 schematically depicts two different ways in which the packets 310a, 310 b could be combined. In a first way, peaks and valleys from highfrequency spectral variations in the reflection (or transmission)spectrum of one packet could align themselves with peaks and valleys,respectively, from high frequency spectral variations in the reflection(or transmission) spectrum of the other packet. One way this could beaccomplished is by fabricating packets 310 a, 310 b to be substantiallyidentical, with substantially identical reflection and transmissionspectra whose high frequency spectral variations—i.e., the various peaksand valleys thereof—are substantially coincident with each other inwavelength. This first way of combining the packets 310 a, 310 bproduces reflected light for the film body 310 whose power per unitwavelength as a function of wavelength is shown in plot 318 a, andtransmitted light whose power per unit wavelength as a function ofwavelength is shown in plot 320 a. The plots 318 a, 320 a are shown tohave a substantial amount of high frequency spectral variability,because the reflection and transmission spectra of the film body 310would include such high frequency variability as a result of thesubstantial alignment of the peaks and valleys associated with theindividual packets 310 a, 310 b.

In a second way of combining the packets 310 a, 310 b, peaks and valleysfrom high frequency spectral variations in the reflection (ortransmission) spectrum of one packet could be substantially misalignedwith peaks and valleys, respectively, from high frequency spectralvariations in the reflection (or transmission) spectrum of the otherpacket. One way this could be accomplished is by initially fabricatingpackets 310 a, 310 b to be substantially identical, with substantiallyidentical reflection and transmission spectra whose high frequencyspectral variations are substantially coincident with each other inwavelength—but then carrying out the additional step of thinning orthickening one of the packets during production very slightly withrespect to the other to shift the reflection and transmission spectra ofone of the packets so that the high frequency spectral peaks of onepacket are substantially misaligned with those of the other packet, andthe high frequency spectral valleys of one packet are substantiallymisaligned with those of the other packet. This second way of combiningthe packets 310 a, 310 b produces reflected light for the film body 310whose power per unit wavelength as a function of wavelength is shownschematically in plot 318 b, and transmitted light whose power per unitwavelength as a function of wavelength is shown schematically in plot320 b. The plots 318 b, 320 b are shown to have a substantially reducedamount of high frequency spectral variability, because the reflectionand transmission spectra of the film body 310 would include such reducedhigh frequency variability as a result of the substantial misalignmentof the peaks and valleys associated with the individual packets 310 a,310 b.

Having now discussed aspects of the invention in broad terms, and beforediscussing those and other aspects in further detail, we turn to FIGS. 4and 5 for a brief review of microlayers, optical repeat units (ORUs),stacks of microlayers, and multilayer optical films and film bodies.

FIG. 4 depicts only two layers of a multilayer optical film 400, whichwould typically include tens or hundreds of such layers arranged in oneor more contiguous packets or stacks. The film 400 includes individualmicrolayers 402, 404, where “microlayers” refer to layers that aresufficiently thin so that light reflected at a plurality of interfacesbetween such layers undergoes constructive or destructive interferenceto give the multilayer optical film the desired reflective ortransmissive properties. The microlayers 402, 404 may together representone optical repeat unit (ORU) of the multilayer stack, an ORU being thesmallest set of layers that recur in a repeating pattern throughout thethickness of the stack. Alternative ORU designs are discussed furtherbelow. The microlayers have different refractive index characteristicsso that some light is reflected at interfaces between adjacentmicrolayers. For optical films designed to reflect light at ultraviolet,visible, or near-infrared wavelengths, each microlayer typically has anoptical thickness (i.e., a physical thickness multiplied by refractiveindex) of less than about 1 μm. Thicker layers can, however, also beincluded, such as skin layers at the outer surfaces of the film, orprotective boundary layers disposed within the film that separatepackets of microlayers.

We may refer to the refractive indices of one of the microlayers (e.g.layer 402 of FIG. 4, or the “A” layers of FIG. 5 below) for lightpolarized along principal x-, y-, and z-axes as n1x, n1y, and n1z,respectively. The mutually orthogonal x-, y-, and z-axes may, forexample, correspond to the principal directions of the dielectric tensorof the material. Typically, and for discussion purposes, the principledirections of the different materials are coincident, but this need notbe the case in general. We refer to the refractive indices of theadjacent microlayer (e.g. layer 404 in FIG. 4, or the “B” layers in FIG.5) along the same axes as n2x, n2y, n2z, respectively. We refer to thedifferences in refractive index between these layers as Δnx (=n1x−n2x)along the x-direction, Δny (=n1y−n2y) along the y-direction, and Δnz(=n1z−n2z) along the z-direction. The nature of these refractive indexdifferences, in combination with the number of microlayers in the film(or in a given stack of the film) and their thickness distribution,controls the reflective and transmissive characteristics of the film (orof the given stack of the film). For example, if adjacent microlayershave a large refractive index mismatch along one in-plane direction (Δnxlarge) and a small refractive index mismatch along the orthogonalin-plane direction (Δny≈0), the film or packet may behave as areflective polarizer for normally incident light. A reflective polarizermay be considered to be an optical body that strongly reflects normallyincident light that is polarized along one in-plane axis, referred to asthe “block axis”, if the wavelength is within the reflection band of thepacket, and strongly transmits such light that is polarized along anorthogonal in-plane axis, referred to as the “pass axis”.

If desired, the refractive index difference (Δn_(z)) between adjacentmicrolayers for light polarized along the z-axis can also be tailored toachieve desirable reflectivity properties for the p-polarizationcomponent of obliquely incident light. To maintain near on-axisreflectivity of p-polarized light at oblique angles of incidence, thez-index mismatch Δn_(z) between microlayers can be controlled to besubstantially less than the maximum in-plane refractive index differenceΔn_(x), such that Δn_(z)≦0.5*Δn_(x). Alternatively, Δn_(z)≦0.25*Δn_(x).A zero or near zero magnitude z-index mismatch yields interfaces betweenmicrolayers whose reflectivity for p-polarized light is constant or nearconstant as a function of incidence angle. Furthermore, the z-indexmismatch Δn_(z) can be controlled to have the opposite polarity comparedto the in-plane index difference Δn_(x), i.e., Δn_(z)<0. This conditionyields interfaces whose reflectivity for p-polarized light increaseswith increasing angles of incidence, as is the case for s-polarizedlight. If Δn_(z)>0, then the reflectivity for p-polarized lightdecreases with angle of incidence. The foregoing relationships also ofcourse apply to relationships involving Δn_(z) and Δn_(y), e.g., incases where significant reflectivity and transmission are desired alongtwo principal in-plane axes (such as a balanced or symmetric partiallyreflecting mirror film, or a partial polarizing film whose pass axis hassignificant reflectivity at normal incidence).

In the schematic side view of FIG. 5, more interior layers of amultilayer film 510 are shown so that multiple ORUs can be seen. Thefilm is shown in relation to a local x-y-z Cartesian coordinate system,where the film extends parallel to the x- and y-axes, and the z-axis isperpendicular to the film and its constituent layers and parallel to athickness axis of the film.

In FIG. 5, the microlayers are labeled “A” or “B”, the “A” layers beingcomposed of one material and the “B” layers being composed of adifferent material, these layers being stacked in an alternatingarrangement to form optical repeat units or unit cells ORU 1, ORU 2, . .. ORU 6 as shown. Typically, a multilayer optical film composed entirelyof polymeric materials would include many more than 6 optical repeatunits if high reflectivities are desired. The multilayer optical film510 is shown as having a substantially thicker layer 512, which mayrepresent an outer skin layer, or a protective boundary layer (“PBL”,see U.S. Pat. No. 6,783,349 (Neavin et al.)) that separates the stack ofmicrolayers shown in the figure from another stack or packet ofmicrolayers (not shown). If desired, two or more separate multilayeroptical films can be laminated together, e.g. with one or more thickadhesive layers, or using pressure, heat, or other methods to form alaminate or composite film.

In some cases, the microlayers can have thicknesses and refractive indexvalues corresponding to a ¼-wave stack, i.e., arranged in ORUs eachhaving two adjacent microlayers of equal optical thickness (f-ratio=50%,the f-ratio being the ratio of the optical thickness of a constituentlayer “A” to the optical thickness of the complete optical repeat unit),such ORU being effective to reflect by constructive interference lightwhose wavelength λ is twice the overall optical thickness of the opticalrepeat unit, where the “optical thickness” of a body refers to itsphysical thickness multiplied by its refractive index. In other cases,the optical thickness of the microlayers in an optical repeat unit maybe different from each other, whereby the f-ratio is greater than orless than 50%. For purposes of the present application, we contemplatemultilayer optical films whose f-ratio may be any suitable value, and donot limit ourselves to films whose f-ratio of 50%. Accordingly, in theembodiment of FIG. 5, the “A” layers are depicted for generality asbeing thinner than the “B” layers. Each depicted optical repeat unit(ORU 1, ORU 2, etc.) has an optical thickness (OT₁, OT₂, etc.) equal tothe sum of the optical thicknesses of its constituent “A” and “B” layer,and each optical repeat unit reflects light whose wavelength λ is twiceits overall optical thickness.

In exemplary embodiments, the optical thicknesses of the ORUs differaccording to a thickness gradient along the z-axis or thicknessdirection of the film, whereby the optical thickness of the opticalrepeat units increases, decreases, or follows some other functionalrelationship as one progresses from one side of the stack (e.g. the top)to the other side of the stack (e.g. the bottom). Such thicknessgradients can be used to provide a widened reflection band to providesubstantially spectrally flat transmission and reflection of light overthe extended wavelength band of interest, and also over all angles ofinterest. Alternatively, the layer thickness gradient of the disclosedpackets of microlayers may be deliberately tailored to providereflection and transmission spectra that change significantly over thewavelength range of interest. For example, it may be desirable for themultilayer optical film body to transmit (or reflect) more blue lightthan red light, or vice versa, or to transmit (or reflect) more greenlight than blue light and red light. Although such desired spectralnon-uniformities may cause the multilayer optical film body to exhibit acolored (non-clear or non-neutral) appearance, this desired color istypically distinguishable from the undesired color discussed elsewhereherein in that the desired color is associated with relatively slowchanges in the spectral reflection or transmission, whereas theundesired color is associated with faster changes in those parameters.For example, spectral non-uniformities in reflection or transmissionassociated with desired color may vary as a function of wavelength withcharacteristic periods of about 100 nm or greater, whereas spectralnon-uniformities in reflection or transmission associated with undesiredcolor may vary as a function of wavelength with characteristic periodsof less than about 50 nm.

To achieve high reflectivities with a reasonable number of layers,adjacent microlayers may exhibit a difference in refractive index (Δnx)for light polarized along the x-axis of at least 0.05, for example. Ifhigh reflectivity is desired for two orthogonal polarizations, then theadjacent microlayers also may exhibit a difference in refractive index(Δny) for light polarized along the y-axis of at least 0.05, forexample. In some cases, adjacent microlayers may have refractive indexmismatches along the two principal in-plane axes (Δnx and Δny) that areclose in magnitude, in which case the film or packet may behave as anon-axis mirror or partial mirror. Alternatively, for reflectivepolarizers that are designed to be partially reflective for the passaxis polarization, adjacent microlayers may exhibit a large differencein refractive index (Δnx) for light polarized along the x-axis and asmaller but still substantial difference in refractive index (Δny) forlight polarized along the y-axis. In variations of such embodiments, theadjacent microlayers may exhibit a refractive index match or mismatchalong the z-axis (Δnz≈0 or Δnz large), and the mismatch may be of thesame or opposite polarity or sign as the in-plane refractive indexmismatch(es). Such tailoring of Δnz plays a key role in whether thereflectivity of the p-polarized component of obliquely incident lightincreases, decreases, or remains the same with increasing incidenceangle.

Although the examples herein describe reflectors whose reflectivityincreases with angle of incidence, partial reflectors whose reflectivityalong a given principal axis decreases with angle of incidence can bemade with reduced color using the same techniques described herein. Thisis particularly important for films whose reflectivity is large atnormal incidence and are viewed in transmitted light at various angles,including normal incidence.

At least some of the microlayers in at least one packet of the disclosedmultilayer optical films may if desired be birefringent, e.g.,uniaxially birefringent or biaxially birefringent, although in someembodiments, microlayers that are all isotropic may also be used. Insome cases, each ORU may include one birefringent microlayer, and asecond microlayer that is either isotropic or that has a small amount ofbirefringence relative to the other microlayer. In alternative cases,each ORU may include two birefringent microlayers.

Exemplary multilayer optical films are composed of polymer materials andmay be fabricated using coextruding, casting, and orienting processes.Reference is made to U.S. Pat. No. 5,882,774 (Jonza et al.) “OpticalFilm”, U.S. Pat. No. 6,179,949 (Merrill et al.) “Optical Film andProcess for Manufacture Thereof”, U.S. Pat. No. 6,783,349 (Neavin etal.) “Apparatus for Making Multilayer Optical Films”, and U.S. PatentApplication 61/332,401 entitled “Feedblock for Manufacturing MultilayerPolymeric Films”, filed May 7, 2010. The multilayer optical film may beformed by coextrusion of the polymers as described in any of theaforementioned references. The polymers of the various layers may bechosen to have similar rheological properties, e.g., melt viscosities,so that they can be co-extruded without significant flow disturbances.Extrusion conditions are chosen to adequately feed, melt, mix, and pumpthe respective polymers as feed streams or melt streams in a continuousand stable manner. Temperatures used to form and maintain each of themelt streams may be chosen to be within a range that avoids freezing,crystallization, or unduly high pressure drops at the low end of thetemperature range, and that avoids material degradation at the high endof the range.

In brief summary, the fabrication method may comprise: (a) providing atleast a first and a second stream of resin corresponding to the firstand second polymers to be used in the finished film; (b) dividing thefirst and the second streams into a plurality of layers using a suitablefeedblock, such as one that comprises: (i) a gradient plate comprisingfirst and second flow channels, where the first channel has across-sectional area that changes from a first position to a secondposition along the flow channel, (ii) a feeder tube plate having a firstplurality of conduits in fluid communication with the first flow channeland a second plurality of conduits in fluid communication with thesecond flow channel, each conduit feeding its own respective slot die,each conduit having a first end and a second end, the first end of theconduits being in fluid communication with the flow channels, and thesecond end of the conduits being in fluid communication with the slotdie, and (iii) optionally, an axial rod heater located proximal to saidconduits; (c) passing the composite stream through an extrusion die toform a multilayer web in which each layer is generally parallel to themajor surface of adjacent layers; and (d) casting the multilayer webonto a chill roll, sometimes referred to as a casting wheel or castingdrum, to form a cast multilayer film. This cast film may have the samenumber of layers as the finished film, but the layers of the cast filmare typically much thicker than those of the finished film. Furthermore,the layers of the cast film are typically all isotropic. A multilayeroptical film with controlled low frequency variations in reflectivityand transmission over a wide wavelength range can be achieved by thethermal zone control of the axial rod heater, see e.g. U.S. Pat. No.6,783,349 (Neavin et al.).

In some cases, the fabrication equipment may employ one or more layermultipliers to multiply the number of layers in the finished film. Inother embodiments, the films can be manufactured without the use of anylayer multipliers. Although layer multipliers greatly simplify thegeneration of a large number of optical layers, they may impartdistortions to each resultant packet of layers that are not identicalfor each packet. For this reason, any adjustment in the layer thicknessprofile of the layers generated in the feedblock is not the same foreach packet, i.e., all packets cannot be simultaneously optimized toproduce a uniform smooth spectrum free of spectral disruptions. Thus, anoptimum profile, for low transmitted and reflected color, can bedifficult to make using multi-packet films manufactured usingmultipliers. If the number of layers in a single packet generateddirectly in a feedblock do not provide sufficient reflectivity, then twoor more such films can be attached to increase the reflectivity. Furtherdiscussion of layer thickness control, so as to provide smooth spectralreflectivity and transmission for low color films, is provided in PCTpublication WO 2008/144656 (Weber et al.).

If the optical thicknesses of all of the microlayers in a givenmultilayer film were designed to be the same, the film would providehigh reflectivity over only a narrow band of wavelengths. Such a filmwould appear highly colored if the band were located somewhere in thevisible spectrum, and the color would change as a function of angle. Inthe context of display and lighting applications, films that exhibitnoticeable colors are generally avoided, although in some cases it maybe beneficial for a given optical film to introduce a small amount ofcolor to correct for color imbalances elsewhere in the system. Exemplarymultilayer optical film bodies are provided with broad band reflectivityand transmission, e.g. over the entire visible spectrum, by tailoringthe microlayers—or more precisely, the optical repeat units (ORUs),which in many (but not all) embodiments correspond to pairs of adjacentmicrolayers—to have a range of optical thicknesses. Typically, themicrolayers are arranged along the z-axis or thickness direction of thefilm from a thinnest ORU on one side of the film or packet to a thickestORU on the other side, with the thinnest ORU reflecting the shortestwavelengths in the reflection band and the thickest ORU reflecting thelongest wavelengths.

After the multilayer web is cooled on the chill roll, it can be drawn orstretched to produce a finished or near-finished multilayer opticalfilm. The drawing or stretching accomplishes two goals: it thins thelayers to their desired final thicknesses, and it may orient the layerssuch that at least some of the layers become birefringent. Theorientation or stretching can be accomplished along the cross-webdirection (e.g. via a tenter), along the down-web direction (e.g. via alength orienter), or any combination thereof, whether simultaneously orsequentially. If stretched along only one direction, the stretch can be“unconstrained” (wherein the film is allowed to dimensionally relax inthe in-plane direction perpendicular to the stretch direction) or“constrained” (wherein the film is constrained and thus not allowed todimensionally relax in the in-plane direction perpendicular to thestretch direction). If stretched along both in-plane directions, thestretch can be symmetric, i.e., equal along the orthogonal in-planedirections, or asymmetric. Alternatively, the film may be stretched in abatch process. In any case, subsequent or concurrent draw reduction,stress or strain equilibration, heat setting, and other processingoperations can also be applied to the film.

FIG. 6 is a graph of a portion of the measured thickness profile 610 ofa multilayer optical film that was manufactured, using the foregoingcoextrusion and stretching processes, to have a single packet of 275microlayers. The microlayers were arranged in a quarter-wave stack withalternating A, B polymer materials, thus providing 137 ORUs in thestack. For the layer thickness values that are shown in the figure, apoint in the profile 610 is plotted for the physical (not optical)thickness of each microlayer, the microlayers being numbered from 1 to250 from one end of the packet to the other. The resulting profile 610is nominally monotonic so as to provide a wide reflection band fromabout 350 to 750 nm for normally incident light, but thicknessvariations relative to an ideal smooth curve can be readily seen. Thesethickness variations exemplify the type of thickness variations that maybe observed in the ORUs of coextruded multilayer optical films. Suchuncontrolled, typically random or quasi-random thickness variations inthe ORUs of the packet give rise to undesired high frequency variationsin the transmission spectrum and reflection spectrum of the film.

Layer profile disruptions or variations such as those shown in FIG. 6may be difficult to eliminate by advanced manufacturing techniques, andfurthermore may not be the same at all positions, points, or areas ofthe film. Thus, in at least some circumstances it may be possible tosubstantially eliminate the thickness variations only for certainportions of the film (which may be manufactured in the form of a longcontinuous web, for example), while other portions of the web exhibitsubstantial variations in the ORU thickness profile. The disruptions inthe layer thickness profile limit the flatness or uniformity of thetransmission and reflection spectra on a small wavelength scale(corresponding to high frequency spectral variations), such as withinany given 50 nm portion of a broadband reflector that extends, e.g.,across the visible spectrum. It can be difficult to provide localizedheating or cooling of the feedblock metal that would be necessary tosmooth out these variations or kinks in the layer profile. We haveobserved that variations or kinks such as this may not appear in acoextruded multilayer optical film reproducibly from run-to-run, and maybe more predominant in the center of the web than near the edges, orvice versa.

The multilayer optical films and film bodies can also include additionallayers and coatings selected for their optical, mechanical, and/orchemical properties. For example, a UV absorbing layer can be added atone or both major outer surfaces of the film to protect the film fromlong-term degradation caused by UV light. Additional layers and coatingscan also include scratch resistant layers, tear resistant layers, andstiffening agents, for example. See, e.g., U.S. Pat. No. 6,368,699(Gilbert et al.).

The materials used in the manufacture of multilayer optical films aretypically polymer materials that have very low absorption at least overvisible and near-visible wavelengths and for typical optical pathdistances within the film. Thus, the percent reflection R and thepercent transmission T of a multilayer film for a given light ray aretypically substantially complementary, i.e., R+T≈1 (or 100%), usuallywithin an accuracy of about 0.01 (1%). Thus, unless otherwise noted, amultilayer optical film disclosed herein as having a certainreflectivity R can be assumed to have a complementary transmission(T=1−R), and vice versa, and reported values of reflectivity ortransmission can be assumed to also report on transmission orreflectivity, respectively, via the relationship R+T≈1 (or 100%).

The reflection and transmission characteristics can be readilydetermined whether one is dealing with a computer-modeled optical filmor an actual film whose properties are measured in the laboratory. Thereflection spectrum and all of its features such as the reflectivity atany angle and the band edges for birefringent multilayer films can becalculated using the 4×4 stack code of Berremen and Scheffer, Phys. Rev.Lett. 25, 577 (1970). A description of this method is given in the book“Ellipsometry and Polarized Light” written by Azzam and Bashara,published by Elsevier Science, Holland.

Multilayer optical films disclosed herein preferably exhibit anintermediate amount of reflectivity, i.e., partial reflection andpartial transmission, over an extended band for one or more specifiedincidence conditions. The partial reflection and partial transmissionprovided by the microlayers over the extended band, e.g., the visiblewavelength range in the case of many display and lighting applications,makes the films susceptible to introducing undesirable color into thesystem if the reflection or transmission characteristic is notsufficiently uniform or smooth as a function of wavelength. Depending onthe system design of which the multilayer optical film is a part, theintermediate reflectivity/transmission may be designed to occur for anydesired incidence condition. In one case, for example, the incidencecondition may be for normally incident light that is unpolarized. Inanother case, the incidence condition may be for normally incident lightpolarized along a block axis or a pass axis of the film. In other cases,the incidence condition may be for light incident obliquely in anyselected plane of incidence, which light may be s-polarized,p-polarized, or a combination thereof.

We now pause to discuss the issue of the perceived “color” associatedwith high frequency spectral variations in reflection or transmission ofa partially reflective multilayer optical film or film body, and how onemay characterize such spectral variations for purposes of the presentapplication.

Color can be measured in many different ways for a given multilayeroptical film body, or for a microlayer packet thereof. In practice, thecolor depends on many factors, including the spectral features of theparticular light source used in the system, whether the film is viewedin reflection or transmission, the angle of incidence of the light, andthe specific caliper of the film stack. Therefore, instead ofcalculating or measuring specific color metrics for all possible lightsources at all possible angles and film calipers, we have found that,for purposes of the present application, it is better to characterize ordescribe the source of undesirable color in the multilayer film in a waythat is independent of the light source or detector. The source ofundesirable color is the multiple narrow or abrupt features in thefilm's transmission and/or reflection spectrum that cannot be removedwith known process or hardware changes. These spectral features are thesmall, relatively high frequency variations in transmission values thattypically occur as 10 nm, 50 nm, or 100 nm wide peaks and valleys in anotherwise smooth spectrum over the wavelength range of interest. Thesehigh frequency variations can give rise to perceivable hues ofiridescent colors in the film in many specific system applications.

An exemplary way to describe the undesired color-producing potential ofa broad band partially reflecting film is to characterize thenon-uniformity or lack of smoothness of the transmission or reflectionspectrum of the film for the specified or desired incidence condition. Asimple and reliable measure of spectral non-uniformity or variability isthe deviation of the spectrum from a smooth spectral target. This can bedone, for example, by taking the difference between a measured (ormodeled) spectrum and a smoothed version of the spectrum. The smoothedversion of the spectrum is preferably more complex than, for example, asingle number equal to the average value of the spectrum averaged overthe wavelength range of interest. It is desirable for the smoothedversion of the spectrum to also include, or to take into account, slowlyvarying spectral features that the film designer may want the film toexhibit, and that the film manufacturer can incorporate into the film byappropriate control of an axial rod heater (in the case of coextrudedmultilayer polymer films) or the like. We have found that, when dealingwith multilayer optical film bodies, or microlayer packets thereof,designed for operation throughout the visible wavelength range, or wherethe wavelength range of interest spans the visible wavelength region andthe near infrared region up to 800, 900, or 1000 nm, for example, thesmoothed version of the spectrum is preferably a curve fit to the actualspectrum, where the curve fit includes only polynomials up to 3^(rd)order in wavelength. Thus, the smoothed version of the transmission orreflection spectrum of a film body or packet thereof may be a best-fitcurve to the actual spectrum over the wavelength range of interest,where the best-fit curve is of the form a₀+a₁λ, +a₂λ²+a₃λ³. The highfrequency spectral variability or non-uniformity, which is the source ofthe undesirable color in the multilayer optical film body or packet, maythen be computed by first taking the difference between the actualspectrum and the smoothed version (the best-fit curve), and thencalculating the standard deviation of this difference curve. Thestandard deviation, or a similar statistical quantity, of such adifference curve can be readily calculated and then used as a measure ofthe high frequency variability of the transmission or reflectionspectrum associated with the undesirable color potential of the film orpacket.

Controlling color in extruded multilayer films involves controlling theextrusion and orientation processes by which the film is made.Techniques disclosed herein for reducing undesirable high frequencycolor may involve controlling the process to make two or more separatemultilayer film stacks and then laminating them together such that atleast some light can pass sequentially through both film stacks. It isoften desirable not only to provide the film body, and individualmicrolayer packets thereof, with a uniform or smoothly varying spectraltransmission or reflection over the wavelength range of interest, butalso to provide the film body and packets thereof with good spatialuniformity, such that the same or similar spectral transmission andreflection is maintained at substantially all possible test points orareas across the active area of the film body or packets. Processes forachieving good spatial uniformity over the active area of a multilayerpolymeric optical film are discussed in U.S. Pat. No. 6,531,230 (Weberet al.), “Color Shifting Film”. Although the '230 patent emphasizesspatial uniformity in multilayer films with abrupt changes in reflectionor transmission as a function of wavelength, such that noticeable “colorshifts” can be seen with changing incidence angle, whereas multilayerfilms of interest to the present application are typically broadbandwith desirably lower perceived color (e.g. a transmission spectrum thatis substantially flat over the wavelength range of interest, or possiblyhaving a moderate slope or curvature over the range of interest), theteachings of the '230 patent with regard to spatial uniformity are stillpertinent to the multilayer films of interest here. Small variations intransmission or reflection which are exhibited as peaks and valleys inan otherwise smooth spectrum behave similar to the sharp band edgesrecited in the '230 patent. Just as with the sharp band edges, the smallvariations in transmission or reflection are more noticeable if the filmbody (or its constituent microlayer packets) is non-uniform in thicknesscaliper, since the wavelength position of a spectral feature is closelycorrelated with the thickness or caliper of the film.

As mentioned above, we have found it feasible to reduce the undesiredcolor of partially transmissive multilayer optical film bodies byutilizing at least two packets of microlayers, each with reflectionspectra for a given incidence condition covering the same or similarbroadband spectral range, and then laminating or otherwise combining themicrolayer packets such that the high frequency spectral peaks andvalleys of one packet tend to cancel the high frequency spectral peaksand valleys of the other packet, resulting in a smoother reflection andtransmission spectrum for the resultant partially transmissivemultilayer optical film body. Spectral peaks of one packet can bealigned with spectral valleys (and misaligned with the spectral peaks)of the other packet by making relatively spatially uniform microlayerpackets (e.g. as distinct films) and making one with a desired caliperdifference from the other. If all of the spectral peaks and valleys of afirst packet or film do not correspond in a one-to-one fashion withthose of a second packet or film, then one can choose the most offendingspectral peak or valley in one packet and align it with an appropriateoffsetting feature in the second packet to reduce the color in the finalproduct.

The disclosed spectral smoothing techniques rely in part on thealignment (including partial alignment) of spectral peaks of onebroadband microlayer packet with spectral valleys of another broadbandmicrolayer packet to produce a smoother transmission and reflectionspectrum in the combined or finished partially transmissive broadbandproduct. An unexpected discovery was that when the high frequency peaksand valleys of one packet do not cancel with those of the other packet,or even if the packets are combined such that the high frequency peaksand valleys of one packet reinforce those of the other packet, the colorof the combined article is not much greater than either one of thesingle packets or films alone. Thus, for example, we found that evenwhere spatial non-uniformities in two microlayer packets are such thatthe high frequency peaks and valleys of one packet can be made to cancelthose of the other packet only at one or more selected points or targetareas on the active area of the combined article, while at other pointsor target areas the high frequency peaks and valleys of one packet donot cancel and may in fact be reinforced by those of the other packet,the resulting combined article may nevertheless exhibit, overall, areduced high frequency color compared to a similar article made withonly a single microlayer packet.

The unexpected color characteristics of the multiple packet multilayeroptical film bodies are in part due to the fact that the reflectivitiesof the constituent microlayer packets of the film body do not addlinearly or coherently, but are combined incoherently according to theso-called “pile-of-plates” formula for the combined reflectivity of tworeflectors R1 and R2:

$\begin{matrix}{R = \frac{{R\; 1} + {R\; 2*\left( {1 - {2\; R\; 1}} \right)}}{1 - {R\; 1*R\; 2}}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$In this equation, R1 and R2 are the reflectivities of the individualmicrolayer packets, and R is the reflectivity of the overall article(film body), which contains both packets. If R1 and R2 are internalreflectivities, then R is the internal reflectivity of the film body.Alternatively, R1 and R2 may collectively incorporate the effects of twoair/polymer interfaces (e.g. R1 may include the effect of oneair/polymer interface, and R2 may include the effect of another suchinterface), whereupon R would represent the external reflectivity of thefilm body. The packets are assumed to be separated by an optically thickmedium, i.e., one that is large compared to the wavelength of light ofinterest. Note that Equation (1) has no wavelength dependence, but thereader will understand that the equation can be evaluated at any givenwavelength of interest. That is, the parameters R, R1, and R2 representthe reflectivities of the film body, the first microlayer packet, andthe second microlayer packet, respectively, all measured at a particularwavelength of interest.

Preferably, the reflectivities R1, R2 of the microlayer packets (ortheir average reflectivities over the wavelength range of interest) arethe same or similar (for example, within a factor of 2), or at least areof about the same order of magnitude. For example, combining a 5%reflective microlayer packet with an 80% reflective microlayer packetdoes little to increase the reflectivity of the film body beyond 80%.Furthermore, the bandwidth of the respective reflection bands for themicrolayer packets are also preferably the same or similar. Thebandwidths and band edges of the packets may be somewhat different, butthe reflection bands of the packets preferably extend over a commonbroad band, e.g., at least the visible spectrum or a major portionthereof.

Preferably, interference effects of air-polymer interfaces (orinterfaces involving air and some other light transmissive opticalmaterial) are substantially avoided in the multilayer optical film bodyso as to keep undesirable high frequency oscillations in thetransmission and reflection spectra to a minimum. For example, otherthan an air-polymer interface at a front and back major surface of thefilm body, the film body may include no other significant air-polymerinterfaces. By incorporating protective boundary layers (PBLs), skinlayers, and/or other optically thick layers or substrates, the film bodyin exemplary embodiments may have an overall physical thickness of atleast 100 micrometers to substantially reduce high frequencyoscillations in the visible portion of the spectrum, and at least 200micrometers to further reduce the high frequency oscillations in thenear infrared portion of the spectrum. In exemplary embodiments, thefirst and second microlayer packets are connected by one or moreoptically thick layers of light-transmissive material of the same orsimilar refractive index as the materials used in the microlayerpackets, with no air gaps between the packets.

Before demonstrating the disclosed high frequency color smoothingprinciples in connection with multiple packet partially reflectivemultilayer optical film bodies of practical design, we demonstrate theprinciples using simple simulations that utilize reflectors(hypothetical microlayer packets) that are characterized bysinusoidally-varying reflection and transmission spectra. These simplesimulations are shown in FIGS. 7a-c . In FIG. 7a , we have plotted thespectral reflectivity 710 of a first hypothetical packet and thespectral reflectivity 712 of a second hypothetical packet. Thesereflectivities have both been modeled to have an average reflectivity of0.30 (30%), with sinusoidal variations therefrom over an extendedwavelength range that ranges from 0 to 400 in arbitrary units. Thesinusoidal variations are assumed to be high frequency and undesired. Inthis case the reflection spectra are intended to be flat, i.e., constantas a function of wavelength, and thus for purposes of characterizing orisolating the undesired high frequency spectral variations in thissimple simulation, we select a flat line (zero order in wavelength) ofreflectivity 0.30 to use as the “smoothed spectrum” for each of thefirst and second hypothetical packets, rather than a 3^(rd) order curvefit as discussed above. Subtracting that flat line from the “actual”reflectivities of the first and second hypothetical packets yields thesinusoidal components of the curves 710, 712, such sinusoidal componentsrepresenting the undesired spectral variability for each of the packets.Note that the two sinusoidal curves are precisely out of phase with eachother, so that peaks in the reflectivity 710 are spectrally aligned withvalleys in the reflectivity 712, and valleys in the reflectivity 710 arespectrally aligned with peaks in the reflectivity 712, and the peaks andvalleys have the same amplitude and width. This represents the optimumcondition for removing high frequency spectral variability from thehypothetical film body. Note that the ripples are not completely reducedto zero, even in the case of equal and opposite amplitudes of the peaksand valleys. We can characterize the high frequency spectral variabilityof the packets as the statistical standard deviation of the sinusoidalcomponents of the respective reflectivity curves. We refer to these highfrequency variability values as Δ1 for the first packet and Δ2 for thesecond packet. In this case, Δ1 and Δ2 are both 0.0354, which are alsolabeled in FIG. 7a next to their respective reflectivity curve.

We can also calculate the mathematical difference between the sinusoidalcomponents of the curves 710, 712, the result of which is a differentialspectrum 714 which extends over the same extended wavelength range. Wecan then calculate the high frequency variability of the differentialspectrum 714, which we refer to as the differential high frequencyvariability Δdiff, e.g., by taking the statistical standard deviation ofthe spectrum 714. In this case, Δdiff is 0.0708, which is labeled in thefigure next to its respective curve.

Finally, we can also calculate, using curves 710 and 712 and Equation(1), the reflectivity of the multilayer optical film body (in this caseequal to the reflectivity of the combination of the first and secondpackets) which would result from laminating the first and secondhypothetical microlayer packets together with an optically thickmaterial layer therebetween. The result is a combination reflectionspectrum 716. Since this combination spectrum is intended to be flat,i.e., constant as a function of wavelength, for purposes ofcharacterizing or isolating the undesired high frequency spectralvariations in this simple simulation, we select a flat line (zero orderin wavelength) to use as the “smoothed spectrum” for the reflectivity ofthe film body or combination, rather than a 3^(rd) order curve fit asdiscussed above. Subtracting that flat line, having a value equal to theaverage reflectivity of curve 716, from the “actual” reflectivity curve716, yields the high frequency spectral variability component of thecurve 716. We can characterize the variability of that curve componentby a single number which we refer to as the combination high frequencyvariability Δcomb, where that number may be calculated as thestatistical standard deviation of the high frequency component of thecurve 716. In this case, Δcomb is 0.0015, and is labeled in FIG. 7a nextto its associated curve 716.

To summarize the high frequency variability values for the hypotheticalembodiment of FIG. 7a , in which the peaks and valleys of thereflectivity spectrum of one microlayer packet are precisely misalignedwith those of the other microlayer packet, we have:

Δ1=0.0354;

Δ2=0.0354;

Δdiff=0.0708; and

Δcomb=0.0015.

This example shows that, when trying to minimize high frequency color(Δcomb) in the film body or combination, it is advantageous to arrangethe microlayer packets such that the high frequency variability (Δdiff)of the differential spectrum is large.

We now modify the hypothetical embodiment of FIG. 7a by shifting thehigh frequency variations of the second microlayer packet so that,rather than the peaks and valleys of one packet being preciselymisaligned with those of the other packet, the peaks and valleys of thetwo packets are precisely aligned. Other relevant aspects of the FIG. 7aembodiment are maintained in the embodiment of FIG. 7b , e.g., theaverage reflectivities of the first and second packets are still both0.30 (30%), except the amplitude of the sinusoidal variations wasincreased slightly for both the first and second packets. Curve 720represents the spectral reflectivity of the first microlayer packet,curve 722 represents the spectral reflectivity of the second microlayerpacket, curve 724 represents the differential spectrum obtained bysubtracting the sinusoidal components of the curves 720, 722, and curve726 is the reflection spectrum of the combination of packets calculatedfrom curves 720 and 722 and Equation (1). The high frequency variabilityvalues Δ1, Δ2, Δdiff, and Δcomb can be calculated in the same way asdescribed in connection with FIG. 7a . The results for the hypotheticalembodiment of FIG. 7b , in which the peaks and valleys of thereflectivity spectrum of one microlayer packet are precisely alignedwith those of the other microlayer packet, are:

Δ1=0.0425;

Δ2=0.0425;

Δdiff=0; and

Δcomb=0.0508.

This example shows that, when trying to minimize high frequency color(Δcomb) in the film body or combination of packets, it isdisadvantageous to arrange the microlayer packets such that the highfrequency variability (Δdiff) of the differential spectrum is small.Note, however, that even though the peaks and valleys of the individualpackets are precisely aligned, corresponding to the worst case scenariofrom a high frequency color standpoint, the color of the film body orcombination, measured in terms of Δcomb, exceeds the color (measured interms of Δ1 and Δ2) of the individual microlayer packets by only amodest amount.

We can now modify the hypothetical embodiment of FIG. 7b by shifting thehigh frequency variations of the second microlayer packet so that,rather than the peaks and valleys of one packet being precisely alignedwith those of the other packet, the peaks and valleys of the two packetsare 90 degrees out of phase, and thus neither precisely aligned norprecisely misaligned. Other relevant aspects of the FIG. 7b embodimentare maintained in the embodiment of FIG. 7c , e.g., the averagereflectivities of the first and second packets are still both 0.30(30%), and the amplitude of the sinusoidal variations is the same. Curve730 represents the spectral reflectivity of the first microlayer packet,curve 732 represents the spectral reflectivity of the second microlayerpacket, curve 734 represents the differential spectrum obtained bysubtracting the sinusoidal components of the curves 730, 732, and curve736 is the reflection spectrum of the combination of packets calculatedfrom curves 730 and 732 and Equation (1). The high frequency variabilityvalues Δ1, Δ2, Δdiff, and Δcomb can be calculated in the same way asdescribed in connection with FIGS. 7a and 7b . The results for thehypothetical embodiment of FIG. 7c , in which the peaks and valleys ofthe reflectivity spectrum of one microlayer packet are neither preciselyaligned nor precisely misaligned with those of the other microlayerpacket, are:

Δ1=0.0425;

Δ2=0.0425;

Δdiff=0.0601; and

Δcomb=0.0356.

This example shows again that, when trying to minimize high frequencycolor (ΔFB1) in the film body or combination of packets, it isadvantageous to arrange the microlayer packets such that the highfrequency variability (Δdiff) of the differential spectrum is large.

We will now show that these same principles can be used in examplesinvolving more realistic or practical multilayer optical films. In somecases, we utilize transmission and/or reflection spectra that weremeasured for a given microlayer packet (e.g. a multilayer optical filmhaving only one microlayer packet, which could then be laminated toanother multilayer optical film of similar construction to produce amulti-packet multilayer optical film body) and/or a given multi-packetmultilayer optical film body. In order to ensure the accuracy andreliability of such spectral measurements, particularly when dealingwith films that may exhibit significantly different spectral propertiesat different positions, points, or areas of the film, it is advantageousto take such transmission and/or reflection measurements over a portionof the film that is small enough so that the measurement does notinadvertently omit, as a result of spatial averaging, the high frequencyvariations that the operator is trying to characterize. A sufficientlysmall portion for reliable measurement purposes is referred to herein asa test area. The test area may be selected such that any given spectralfeature of the film body shifts in wavelength by less than a givenamount, e.g., 1 nm, or 2 nm, or 5 nm, between any two portions of thetest area. Note that depending on the degree of spatial uniformity ofthe film sample, the test area may be made relatively large (if desired)in some cases, but may need to be substantially smaller in other cases.

Several exemplary test areas are shown in FIG. 8. In that figure, amultilayer optical film 810, which may be a single packet film that willlater be laminated or otherwise joined to another multilayer opticalfilm, or which may be a multi-packet multilayer optical film body, isshown in perspective view. The film 810 may be in the form of a flexiblepolymer web, suitable for storing in roll form, for example. The film810 is partially reflective over a broad wavelength range for lightpolarized along at least one major axis (principal axis) of the film,and has at least one characteristic spectral feature (e.g., a peak,valley, or transition in the reflection or transmission spectrum) whichcan be measured, for example with a spectrophotometer. Due to filmcaliper variations or for other reasons, the wavelength at which thecharacteristic spectral feature appears is a function of position on thesurface or useable area of the film. Lines 812 a-f depict places on thefilm where the spectral feature appears at a given wavelength: along theline 812 c, the feature appears at a wavelength λ₀; along the line 812a, the spectrum has shifted such that the feature appears at awavelength λ₀+10 (e.g., if wavelength λ₀ is 600 nm, then along line 812a the feature appears at 610 nm); along the line 812 f, the spectrum hasshifted such that the feature appears at a wavelength λ₀-15 (585 nm ifλ₀ is 600 nm); and so forth. The lines 812 a-f are thus analogous tolines on a contour map. Areas 814, 816 are exemplary test areas that onemay use to measure the spectral transmission or reflection at theindicated places on the film 810, if one wished to have better than a 5nm measurement resolution. Of course, a more stringent (or more lax)requirement on measurement resolution would entail a smaller (or larger)test area. Note that the test area need not be circular, and may beelongated if the spatial uniformity of the film is better in onein-plane direction (e.g. the down-web direction) than in anotherin-plane direction (e.g. the cross-web direction).

Turning now to FIG. 9a , we see there measured spectral propertiesassociated with a 2-packet reflective polarizing film body. Eachmicrolayer packet of the film body had 275 microlayers, the microlayersbeing arranged in an alternating A, B pattern with a high refractiveindex polymer material of 90/10 coPEN (a copolyester containing 90%ethylene naphthalate repeat units and 10% ethylene teraphthalate repeatunits), and a low refractive index polymer material of PETg availablefrom Eastman Chemical Company. These materials were coextruded, fedthrough a die, and cast onto a casting wheel (see e.g. U.S. Pat. No.6,783,349 (Neavin et al.) and U.S. Patent Application 61/332,401, filedMay 7, 2010, mentioned above). The cast web was then stretched about2.5:1 in a length orienter, and about 6:1 in a tenter. The twomicrolayer packets, which were separated by an optically thick polymerlayer, each had a nominally monotonic thickness gradient correspondingto a partial reflection band extending from 400 nm to about 1100 nm fornormally incident light polarized along the pass axis of the film body.The polarizing film had a significant amount of reflectivity overvisible and near IR wavelengths for normally incident light polarizedalong the in-plane pass axis, as well as for obliquely incident light.

The transmission of the 2-packet film body was measured at one locationor test area on the film for normally incident light polarized along thepass axis, and the measured transmission is shown as curve 910. The testarea (about 5 mm in diameter) was marked for later reference on bothsides of the film body. The two microlayer packets of the film body werethen separated from each other by peeling the film body apart whilemaintaining the individual microlayer packets intact. One of themicrolayer packets, referred to here as the first microlayer packet, wasadhered to a clear release liner using a layer of optically clearadhesive. This was done to reduce spectral ringing at infraredwavelengths. The marks defining the previously measured test area of thefilm body were used to locate the same location on the first and secondmicrolayer packets, and the transmission of the packets were measuredindividually in the same way as the original 2-packet film body. Curve912 shows the measured transmission for the first packet, and curve 914shows the measured transmission for the second packet.

Each of the curves 910, 912, 914 was then approximated by a smoothed,best-fit curve of the form a₀+a₁λ, +a₂λ²+a₃λ³ between the limits of 400and 1000 nm. These slowly varying best-fit curves are drawn in FIG. 9awith narrow linewidths, but are not labeled. (Similar non-labeledbest-fit curves are also shown in FIGS. 10, 11, and 12 a-d.) In eachcase, the best-fit curve was then subtracted from the measuredtransmission spectrum to isolate the high frequency component of themeasured spectrum. The standard deviation of the high frequencycomponents for each of the curves 910, 912, 914 was calculated betweenthe limits of 400 and 850 nm, the results being ΔFB=Δcomb=0.0226,Δ1=0.0200, and Δ2=0.0362, respectively. The transmission spectra of thefirst and second packets were also subtracted from each other tocalculate a differential spectrum, which is plotted as curve 916. Thestandard deviation of the differential spectrum 916, after subtracting a3^(rd) order best fit curve, was calculated to be Δdiff=0.0456 betweenthe limits of 400 and 850 nm. In summary, then, we have for thisexample:

Δ1=0.0200;

Δ2=0.0362;

Δdiff=0.0456; and

Δcomb=0.0226.

We performed some additional analysis to confirm that Equation (1) isreliable for predicting the spectra of optical film laminates. Inparticular, we used the measured transmission spectra for the first andsecond packets, i.e., curves 912 and 914 in FIG. 9a , to simulate alaminate of those two packets. If our methodology is reliable, thetransmission spectrum we calculate for the simulated laminate shouldmatch the measured transmission spectrum 910 of the actual laminate asit existed before the peeling operation, which was used to isolate thepackets. In our analysis, special consideration was given to the effectsof the film/air interfaces. The original film body or laminate had twoair interfaces, while the two individual packets (in the form ofseparate, peeled films) have a combined total of 4 film/air interfaces.Two of these air interfaces needed to be mathematically removed beforethe pile-of-plates formula above (Equation (1)) can be used to calculatethe transmission of the simulated laminate, keeping in mind that T=1−R.To mathematically remove the excess air interfaces, the reflectivity ofone air/PETg interface (refractive index of PETg is 1.564) wascalculated as a function of wavelength, with dispersion effectsincluded. The “internal reflectivity” of the first packet was thencalculated by the following formula:

$\begin{matrix}{{A_{int} = \frac{1 - {air} - {\left( {1 + {air}} \right)*A}}{1 - {air} - {2*{air}*A}}},} & {{Equation}\mspace{14mu}(2)}\end{matrix}$

where A_(int) refers to the “internal reflectivity” of the first packet,“air” refers to the calculated reflectivity of the air/PETg interface,“A” refers to the measured total transmission of the peeled filmcontaining the first packet (which included the effects of twoair/polymer interfaces), and the relationship R=1−T is assumed for allcalculations. Note that the term “internal reflectivity” of a film orother body refers to the reflectivity the body would have in the absenceof any interfaces at the front or back (or top or bottom, etc.) of thebody due to contact with air or any other medium of different refractiveindex than the film. The term “internal transmission” analogously refersto the transmission the body would have in the absence of any interfacesat the front or back (or top or bottom, etc.) of the body.

After calculating the internal reflectivity A_(int) of the first packet,we then calculated the transmission of the simulated laminate usingEquation (1) above for each wavelength value, where the value of A_(int)was used for “R1” in Equation (1), and the measured reflectivity of thesecond packet (which included the effects of two air/polymer interfaces)was used for “R2” in Equation (1), and where transmission T=1−R. Theresulting calculated transmission spectrum for the simulated laminate isshown as curve 910 a in FIG. 9b , and is plotted alongside the originalmeasured transmission of curve 910. As the reader can see, excellentagreement is achieved between the measured and calculated reflectivitiesof the laminate, confirming that the transmission spectra 912, 914obtained for the individual microlayer packets for the embodiment ofFIG. 9a are reliable and that our use of Equation (1) is valid fordescribing the reflectivity of incoherent reflective packets.

FIG. 10 shows measured spectral properties associated with another2-packet reflective polarizing film body. Each microlayer packet of thefilm body had 275 microlayers, the microlayers being arranged in analternating A, B pattern with one polymer material of polyethyleneterephthalate (PET), and the other polymer material of the PETg polymermentioned above. These materials were coextruded, fed through a die, andcast onto a casting wheel (see e.g. U.S. Pat. No. 6,783,349 (Neavin etal.) and U.S. Patent Application 61/332,401, filed May 7, 2010,mentioned above). The cast web was then stretched about 6:1 in a tenterto produce a broadband reflective polarizer multilayer optical filmbody. The two microlayer packets, which were connected to each other byan optically thick polymer layer, each had a nominally monotonicthickness gradient corresponding to a reflection band extending from 400nm to about 850 nm for normally incident light polarized along the blockaxis of the film body. Like the reflective polarizer of FIGS. 9a-b ,this reflective polarizing film body also had significant reflectivityover visible and near IR wavelengths for normally incident lightpolarized along the pass axis, and also for obliquely incident light.Thus, for light linearly polarized along the pass axis, including bothnormally incident light and obliquely incident light incident in a “passplane” (a plane containing the surface normal and the pass axis of thefilm body) and p-polarized in the plane of incidence, a partialreflectivity over a broad wavelength range was observed.

The transmission spectrum of the film body was measured in a test areaof the film body for p-polarized light obliquely incident in the passplane at an angle of 60 degrees (measured relative to the surfacenormal, in air). The measured transmission spectrum for the film body isshown as curve 1010 in FIG. 10. The film body was then peeled apart toisolate the two microlayer packets from each other, in a manner similarto that described in connection with FIGS. 9a-b . The transmissionspectra of the two packets were measured at the same test area as theoriginal film body, and using the same incidence condition ofp-polarized light incident in the pass plane at 60 degrees. Thetransmission of the first and second packets measured in this way areshown as curves 1012, 1014, respectively.

Each of the curves 1010, 1012, 1014 was then approximated by a smoothed,best-fit curve of the form a₀+a₁λ, +a₂λ²+a₃λ³ between the limits of 400and 750 nm. In each case, the best-fit curve was then subtracted fromthe measured transmission spectrum to isolate the high frequencycomponent of the measured spectrum. The standard deviation of the highfrequency components for each of the curves 1010, 1012, 1014 wascalculated between the limits of 400 and 700 nm, the results beingΔFB=Δcomb=0.0107, Δ1=0.0155, and Δ2=0.0132, respectively. Thetransmission spectra of the first and second packets were alsosubtracted from each other to calculate a differential spectrum, whichis plotted as curve 1016. The standard deviation of the differentialspectrum 1016, after subtracting a 3^(rd) order best fit curve, wascalculated to be Δdiff=0.0185 between the limits of 400 and 700 [?] nm.In summary, then, we have for this example:

Δ1=0.0155;

Δ2=0.0132;

Δdiff=0.0185; and

Δcomb=0.0107.

The transmission spectra measured for the two packets individually werevirtually the same, differing only in the high frequency variationscaused by the uncontrolled disruptions of the layer profile. The highfrequency variability of the film body or laminate can clearly be seento be smaller than that of either packet alone.

FIG. 11 shows measured spectral properties associated with another2-packet multilayer optical film body. In this case, however, themicrolayer packets were fabricated as separate films, and then laminatedtogether to form the film body. A base multilayer optical film was thusconstructed which had only one microlayer packet of 275 microlayers, themicrolayers being arranged in an alternating A, B pattern with onepolymer material being the 90/10 coPEN mentioned above, and the otherpolymer material being a blend of the 90/10 coPEN and PETg in a 55/45ratio. The polymer blend exhibits a refractive index of 1.595. Thesematerials were coextruded, fed through a die, and cast onto a castingwheel (see e.g. U.S. Pat. No. 6,783,349 (Neavin et al.)). The cast webwas then biaxially oriented in an asymmetrical fashion, using a 4.0:1length orientation and about a 6:1 tenter stretch ratio at 142 degreesC. in the width direction, followed by a high temperature heat set at232 degrees C., to produce a broadband single packet multilayer opticalfilm. Two pieces of this single packet base film were cut from the webat different cross-web locations on the film roll, which provided afirst and second film having similar first and second microlayerpackets, respectively. The transmission of each of these film pieces wasmeasured using normally incident light that was linearly polarized alongthe same principal in-plane axis of the films, and the results are shownby curves 1112, 1114 in FIG. 11. These spectra are similar, but spectralfeatures are offset or shifted in wavelength as a result of a minorcaliper difference in the original base film from which the pieces werecut. Besides exhibiting substantial reflectivity at normal incidence,the films also exhibited substantial reflectivity for p-polarized lightincident at 60 degrees in a plane of incidence that includes theprincipal in-plane axis.

The two film pieces were then laminated together (with their respectiveprincipal axes parallel to each other) with a clear pressure sensitiveadhesive (PSA) to produce a 2-packet, broadband, partially reflectingmultilayer optical film body or laminate. The transmission of this filmbody was measured in the same was as the individual pieces, and theresulting measurements are given by curve 1110 in FIG. 11.

Each of the curves 1110, 1112, 1114 was then approximated by a smoothed,best-fit curve of the form a₀+a₁λ, +a₂λ²+a₃λ³ between the limits of 400and 850 nm. In each case, the best-fit curve was then subtracted fromthe measured transmission spectrum to isolate the high frequencycomponent of the measured spectrum. The standard deviation of the highfrequency components for each of the curves 1110, 1112, 1114 wascalculated between the limits of 400 and 850 nm, the results beingΔFB=Δcomb=0.0172, Δ1=0.0288, and Δ2=0.0312, respectively. Thetransmission spectra of the first and second packets were alsosubtracted from each other to calculate a differential spectrum, whichis plotted as curve 1116. The standard deviation of the differentialspectrum 1116, after subtracting a 3^(rd) order best fit curve, wascalculated to be Δdiff=0.0482 between the limits of 400 and 850 nm. Insummary, then, we have for this example:

Δ1=0.0288;

Δ2=0.0312;

Δdiff=0.0482; and

Δcomb=0.0172.

Here, each of the packets has a relatively large high frequencyvariability, compared to the substantially lower value for the film bodyor laminate.

Two more single packet partially reflecting broadband multilayer opticalfilms were fabricated and their spectral properties measured.Computational analysis was then performed on the measured spectra, insuch a way that several different laminated constructions or film bodieswere simulated (but not actually fabricated). The computational analysisallowed us to simulate making one of the single packet films slightlythicker or thinner, causing the transmission spectrum of that film toshift to longer or shorter wavelengths, respectively. The effects ofthose shifts on the high frequency variability of the resultant(simulated) 2-packet film body could then be modeled and analyzed. Theresults are provided in FIGS. 12a-d . The point of this exercise is todemonstrate that separate multilayer optical films being made in thefactory can be adjusted on-line or on-the-fly to produce a multi-packetlaminate or film body that can have reduced high frequency spectralvariability relative to its constituent films.

Further in regard to FIGS. 12a-d , a base multilayer optical film wasconstructed which had only one microlayer packet of 275 microlayers, themicrolayers being arranged in an alternating A, B pattern with onepolymer material being the 90/10 coPEN mentioned above, and the otherpolymer material being the 55/45 blend of 90/10 coPEN and PETg mentionedabove. These materials were coextruded, fed through a die, and cast ontoa casting wheel (see e.g. U.S. Pat. No. 6,783,349 (Neavin et al.)). Thecast web was then biaxially oriented in an asymmetrical fashion, using a3.5:1 length orientation and about a 6:1 tenter stretch ratio at 142degrees C. in the width direction, to produce a broadband single packetmultilayer optical film. Two pieces of this single packet base film werecut from the web at about the same cross-web position but at differentdown-web positions on the film roll, which provided a first and secondfilm having similar first and second microlayer packets, respectively.Each film had significant reflectivity along an in-plane pass axis(principal axis) thereof. The transmission of each of these film pieceswas measured using normally incident light that was linearly polarizedalong the pass axis of the film, and the results are shown by curves1212 a, 1214 a in FIG. 12a . As can be seen, these films exhibitedpartial reflectivity along the pass axis, with a broad reflection bandextending from 400 nm to about 1250 nm at normal incidence.

A first simulated laminate or film body was then evaluated using thecurves 1212 a, 1214 a, in a manner analogous to that described inconnection with FIG. 9b . The calculated transmission of the first filmbody, assuming the same incidence condition used for each of the curves1212 a, 1214 a, is shown as curve 1210 a in FIG. 12a . This curve isbelieved to be representative of the transmission one would measure on afilm body made by laminating the individual film pieces together, underthe given incidence condition.

Each of the curves 1210 a, 1212 a, 1214 a was then approximated by asmoothed, best-fit curve of the form a₀+a₁λ, +a₂λ²+a₃λ³ between thelimits of 400 and 1150 nm. In each case, the best-fit curve was thensubtracted from the measured or calculated transmission spectrum toisolate the high frequency component of the measured/calculatedspectrum. The standard deviation of the high frequency components foreach of the curves 1210 a, 1212 a, 1214 a was calculated between thelimits of 400 and 950 nm, the results being ΔFB=Δcomb=0.0261, Δ1=0.0269,and Δ2=0.0231, respectively. The transmission spectra of the first andsecond packets were also subtracted from each other to calculate adifferential spectrum, which is plotted as curve 1216 a. The standarddeviation of the differential spectrum 1216 a, after subtracting a3^(rd) order best fit curve, was calculated to be Δdiff=0.0277 betweenthe limits of 400 and 950 nm. In summary, then, we have for thisexample:

Δ1=0.0269;

Δ2=0.0231;

Δdiff=0.0277; and

Δcomb=0.0261.

A second simulated laminate or film body was then evaluated. This secondsimulated laminate was substantially the same as the first simulatedlaminate (FIG. 12a ), except that the spectrum of the first singlepacket film (see curve 1212 a in FIG. 12a ) was shifted in wavelength torepresent a 3% increase in the thickness of the first single packetfilm. The calculated transmission spectrum of the thickened first packetis shown in FIG. 12b as curve 1212 b. The curve 1214 b in FIG. 12b isidentical to curve 1214 a in FIG. 12a , and curve 1210 b represents thecombination of curves 1212 b and 1214 b using Equation (1).

Just as before, each of the curves 1210 b, 1212 b, 1214 b was thenapproximated by a smoothed, best-fit curve of the form a₀+a₁λ,+a₂λ²+a₃λ³ between the limits of 400 and 1150 nm, and the best-fit curvewas subtracted from the respective measured or calculated transmissionspectrum to isolate the high frequency component. The standard deviationof the high frequency components for each of the curves 1210 b, 1212 b,1214 b was calculated between the limits of 400 and 950 nm, the resultsbeing ΔFB=Δcomb=0.0203, Δ1=0.0279, and Δ2=0.0231, respectively. Thetransmission spectra of the first and second packets were againsubtracted from each other to calculate a differential spectrum, whichis plotted as curve 1216 b. The standard deviation of the differentialspectrum 1216 b, after subtracting a 3^(rd) order best fit curve, wascalculated to be Δdiff=0.0404 between the limits of 400 and 950 nm. Insummary, then, we have for this example:

Δ1=0.0279;

Δ2=0.0231;

Δdiff=0.0404; and

Δcomb=0.0203.

A third simulated laminate or film body was then evaluated. This thirdsimulated laminate was substantially the same as the first simulatedlaminate (FIG. 12a ), except that the spectrum of the first singlepacket film (see curve 1212 a in FIG. 12a ) was shifted in wavelength torepresent a 2% decrease in the thickness of the first single packetfilm. The calculated transmission spectrum of the thinned first packetis shown in FIG. 12c as curve 1212 c. The curve 1214 c in FIG. 12c isidentical to curve 1214 a in FIG. 12a , and curve 1210 c represents thecombination of curves 1212 c and 1214 c using Equation (1).

Just as before, each of the curves 1210 c, 1212 c, 1214 c was thenapproximated by a smoothed, best-fit curve of the form a₀+a₁λ,+a₂λ²+a₃λ³ between the limits of 400 and 1150 nm, and the best-fit curvewas subtracted from the respective measured or calculated transmissionspectrum to isolate the high frequency component. The standard deviationof the high frequency components for each of the curves 1210 c, 1212 c,1214 c was calculated between the limits of 400 and 950 nm, the resultsbeing ΔFB=Δcomb=0.0286, Δ1=0.0260, and Δ2=0.0231, respectively. Thetransmission spectra of the first and second packets were againsubtracted from each other to calculate a differential spectrum, whichis plotted as curve 1216 c. The standard deviation of the differentialspectrum 1216 c, after subtracting a 3^(rd) order best fit curve, wascalculated to be Δdiff=0.0160 between the limits of 400 and 950 nm. Insummary, then, we have for this example:

Δ1=0.0260;

Δ2=0.0231;

Δdiff=0.0160; and

Δcomb=0.0286.

Note that although nearly identical spectra are being combined, thestandard deviation of the laminate is only a little higher than that ofeach individual film.

The first simulated film body of FIG. 12a was then re-evaluated for adifferent incidence condition. Rather than normally incident lightlinearly polarized along the pass axis of the film, we now used anincidence condition in which the light is obliquely incident in the passplane at 60 degrees, the light being polarized in the plane of incidence(p-polarized). The transmission of the first and second single packetfilms was measured at this new incidence condition, the results plottedas curves 1212 d and 1214 d, respectively, in FIG. 12 d.

The simulated film body was then evaluated using the curves 1212 d, 1214d, in a manner analogous to that described in connection with FIG. 12a .The calculated transmission of the simulated film body, assuming thesame (oblique) incidence condition used for each of the curves 1212 d,1214 d, is shown as curve 1210 d in FIG. 12d . This curve is believed tobe representative of the transmission one would measure on a film bodymade by laminating the individual film pieces together, under the given(oblique) incidence condition. Air interfaces were ignored for thiscalculation since the surface reflectivity is negligible near theBrewster angle for p-polarized light.

Each of the curves 1210 d, 1212 d, 1214 d was then approximated by asmoothed, best-fit curve of the form a₀+a₁λ, +a₂λ²+a₃λ³ between thelimits of 400 and 950 nm, and the best-fit curve was subtracted from therespective measured or calculated transmission spectrum to isolate thehigh frequency component. The standard deviation of the high frequencycomponents for each of the curves 1210 d, 1212 d, 1214 d was calculatedbetween the limits of 400 and 800 nm, the results beingΔFB=Δcomb=0.0176, Δ1=0.0219, and Δ2=0.0220, respectively. Thetransmission spectra of the first and second packets were againsubtracted from each other to calculate a differential spectrum, whichis plotted as curve 1216 d. The standard deviation of the differentialspectrum 1216 d, after subtracting a 3^(rd) order best fit curve, wascalculated to be Δdiff=0.0141 between the limits of 400 and 800 nm. Insummary, then, we have for this example:

Δ1=0.0219;

Δ2=0.0220;

Δdiff=0.0141; and

Δcomb=0.0176.

From the foregoing examples and teachings, one can see that the way inwhich broadband, partially reflective microlayer packets areincorporated into a multi-packet film body can be important from thestandpoint of color in the film body, particularly, color associatedwith undesirable high frequency variability in the reflection and/ortransmission spectra. Desirably, Δdiff is greater than at least one ofΔ1 and Δ2 at least at a first test area of the film body. (Theembodiment of FIG. 12d is somewhat of a counterexample, because Δcomband ΔFB are smaller than either of Δ1 or Δ2, even though the differencevariability Δdiff is small.) Δdiff may also be greater than each of Δ1and Δ2. Furthermore, ΔFB (and/or Δcomb) may be less than at least one ofΔ1 and Δ2, or less than each of Δ1 and Δ2 at the first test area. In asecond test area of the film body, the quantities Δdiff, Δ1, Δ2, ΔFB,and Δcomb may be referred to as Δdiff2, Δ3, Δ4, ΔFB2, and Δcomb2,respectively, and Δdiff2 may be less than at least one of Δ3 and Δ4, orless than each of Δ3 and Δ4. Furthermore, ΔFB2 and/or Δcomb2 may begreater than at least one of Δ3 and Δ4, or it or they may be greaterthan both Δ3 and Δ4, or it or they may be less than one or both of Δ3and Δ4, while ΔFB and/or Δcomb may be less than at least one of Δ1 andΔ2, or less than each of Δ1 and Δ2.

Unless otherwise indicated, all numbers expressing quantities,measurement of properties, and so forth used in the specification andclaims are to be understood as being modified by the term “about”.Accordingly, unless indicated to the contrary, the numerical parametersset forth in the specification and claims are approximations that canvary depending on the desired properties sought to be obtained by thoseskilled in the art utilizing the teachings of the present application.Not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Notwithstandingthat the numerical ranges and parameters setting forth the broad scopeof the invention are approximations, to the extent any numerical valuesare set forth in specific examples described herein, they are reportedas precisely as reasonably possible. Any numerical value, however, maywell contain errors associated with testing or measurement limitations.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the spirit and scopeof this invention, and it should be understood that this invention isnot limited to the illustrative embodiments set forth herein. Forexample, the reader should assume that features of one disclosedembodiment can also be applied to all other disclosed embodiments unlessotherwise indicated. It should also be understood that all U.S. patents,patent application publications, and other patent and non-patentdocuments referred to herein are incorporated by reference, to theextent they do not contradict the foregoing disclosure.

The invention claimed is:
 1. A partially reflective multilayer opticalfilm body having a first principal in-plane axis, comprising: a firstpacket of microlayers having a nominally monotonic layer profile; and asecond packet of microlayers having a nominally monotonic layer profileand connected to the first packet such that at least some light can passthrough the first and second packets of microlayers sequentially;wherein the first and second packets are each configured to partiallytransmit and partially reflect, over an extended wavelength range,normally incident light linearly polarized along the first principalin-plane axis; wherein the first and second packets, in combination,have a first combined internal transmission in a range from 0.05 (5%) to0.95 (95%) for the normally incident light when averaged over theextended wavelength range; wherein the first and second packets, incombination, have a second combined internal transmission for obliquelight that is (a) incident at 60 degrees in a first principal planecontaining the first principal in-plane axis, and (b) linearly polarizedin the first principal plane, the second combined internal transmissionbeing in a range from 0.1 (10%) to 0.9 (90%) when averaged over theextended wavelength range; wherein, at least in a first test area of themultilayer optical film body, a combined high frequency spectralvariability (Δcomb) of the first and second packets in combination isless than a first high frequency spectral variability (Δ1) of the firstpacket by itself; and wherein a reflection band of the first packet anda reflection band of the second packet overlap by at least 70%.
 2. Thefilm body of claim 1, wherein in at least the first test area, thecombined high frequency spectral variability is also less than a secondhigh frequency spectral variability (Δ2) of the second packet by itself.3. The film body of claim 2, wherein the first high frequency spectralvariability Δ1, the second high frequency spectral variability Δ2, andthe combined high frequency spectral variability Δcomb are all evaluatedfor the normally incident light over the extended wavelength range. 4.The film body of claim 3, wherein the extended wavelength rangecomprises at least a majority of a range from 400 to 700 nm.
 5. The filmbody of claim 4, wherein the extended wavelength range extends from 420nm to 680 nm.
 6. The film body of claim 4, wherein the extendedwavelength range extends from 420 nm to a wavelength greater than 680nm.
 7. The film body of claim 2, wherein the first high frequencyspectral variability Δ1, the second high frequency spectral variabilityΔ2, and the combined high frequency spectral variability Δcomb are allevaluated for the obliquely incident light over the extended wavelengthrange.
 8. The film body of claim 1, wherein, at least in the first testarea: the first packet of microlayers exhibits a first transmissionspectrum over the extended wavelength range for the normally incidentlight, the first transmission spectrum having the first high frequencyspectral variability Δ1; the second packet of microlayers exhibits asecond transmission spectrum over the extended wavelength range for thenormally incident light, the second transmission spectrum having thesecond high frequency spectral variability Δ2; a difference between thefirst and second transmission spectra yields a first differentialtransmission spectrum over the extended wavelength range, the firstdifferential transmission spectrum having a first differential highfrequency spectral variability (Δdiff); and the first differential highfrequency spectral variability Δdiff is greater than at least one of thefirst high frequency spectral variability Δ1 and the second highfrequency spectral variability Δ2.
 9. The film body of claim 8, whereinthe first differential high frequency spectral variability Δdiff isgreater than each of the first high frequency spectral variability Δ1and the second high frequency spectral variability Δ2.
 10. The film bodyof claim 8, wherein the first high frequency spectral variability Δ1 isbased on a difference between the first transmission spectrum and afirst best-fit curve to the first transmission spectrum over thewavelength range of interest, the first best-fit curve being of the forma₀+a₁λ, +a₂λ²+a₃λ³.
 11. The film body of claim 8, wherein the reflectionband of the first packet and the reflection band of the second packeteach extend over a major portion of a visible spectrum.
 12. The filmbody of claim 11, wherein the reflection band of the first packet andthe reflection band of the second packet each extend over at least thevisible spectrum.
 13. The film body of claim 8, wherein the partiallyreflective multilayer film body is a two packet multilayer optical filmbody.
 14. The film body of claim 8, wherein: the first high frequencyspectral variability Δ1 is based on a difference between the firstinternal transmission spectrum and a first best-fit curve to the firstinternal transmission spectrum over the wavelength range of interest,the first best-fit curve being of the form a₀+a₁λ, +a₂λ²+a₃λ³; and thesecond high frequency spectral variability Δ2 is based on a differencebetween the second internal transmission spectrum and a second best-fitcurve to the second internal transmission spectrum over the wavelengthrange of interest, the second best-fit curve also being of the forma₀+a₁λ, +a₂λ²+a₃λ³; the first differential high frequency spectralvariability Δdiff is based on a difference between the firstdifferential transmission spectrum and a first differential best-fitcurve to the first differential transmission spectrum over thewavelength range of interest, the first differential best-fit curve alsobeing of the form a₀+a₁λ, +a₂λ²+a₃λ³; the first and second packets incombination exhibit a first combination transmission spectrum over theextended wavelength range for the normally incident light, the firstcombination transmission spectrum having the combined high frequencyspectral variability Δcomb; and the combined high frequency spectralvariability Δcomb is based on a difference between the first combinationtransmission spectrum and a first combination best-fit curve to thefirst combination transmission spectrum over the wavelength range ofinterest, the first combination best-fit curve also being of the forma₀+a₁λ, +a₂λ²+a₃λ³.
 15. The film body of claim 14, wherein: the firsthigh frequency spectral variability Δ1 is a standard deviation of thedifference between the first internal transmission spectrum and thefirst best-fit curve; the second high frequency spectral variability Δ2is a standard deviation of the difference between the second internaltransmission spectrum and the second best-fit curve; the firstdifferential high frequency spectral variability Δdiff is a standarddeviation of the difference between the first differential transmissionspectrum and the first differential best-fit curve; and the combinedhigh frequency spectral variability Δcomb is a standard deviation of thedifference between the first combination transmission spectrum and thefirst combination best-fit curve.
 16. The film body of claim 1, whereinthe film body is a reflective polarizer, and wherein the first principalin-plane axis is a pass axis of the reflective polarizer.
 17. The filmbody of claim 1, wherein the film body is a partial reflector havingsubstantially the same reflectivity for normally incident lightpolarized along the first principal in-plane axis as for normallyincident light polarized along a second in-plane axis perpendicular tothe first principal in-plane axis.
 18. The film body of claim 1, whereinthe first test area is selected such that any given spectral feature ofthe film body shifts in wavelength by less than 1 nm between any twoportions of the first test area.
 19. The film body of claim 1, wherein,in at least a second test area of the multilayer optical film body, asecond combined high frequency spectral variability (Δcomb2) of thefirst and second packets in combination is greater than at least one ofa third high frequency spectral variability (Δ3) of the first packet byitself and a fourth high frequency spectral variability (Δ4) of thesecond packet by itself.
 20. The film body of claim 19, wherein, in thesecond test area, a difference spectrum between a transmission spectrumof the first packet and a transmission spectrum of the second packet hasa second differential high frequency spectral variability Δdiff2, andwherein the second differential high frequency spectral variabilityΔdiff2 is less than at least one of the third high frequency spectralvariability Δ3 and the fourth high frequency spectral variability Δ4.21. The film body of claim 1, wherein the reflection band of the firstpacket and the reflection band of the second packet each extend over amajor portion of a visible spectrum.
 22. The film body of claim 21,wherein the reflection band of the first packet and the reflection bandof the second packet each extend over at least the visible spectrum. 23.The film body of claim 1, wherein the partially reflective multilayerfilm body is a two packet multilayer optical film body.
 24. The filmbody of claim 1, wherein the partially reflective multilayer film bodyincludes no microlayer packets other than the first and second packets.25. The film body of claim 1, wherein the reflection band of the firstpacket and the reflection band of the second packet overlap by at least80%.
 26. The film body of claim 1, wherein the reflection band of thefirst packet and the reflection band of the second packet overlap by atleast 90%.
 27. A method of making a partially reflective multilayeroptical film body, comprising: providing a first and second packet ofmicrolayers each having a nominally monotonic layer profile and,configured to partially transmit and partially reflect, over an extendedwavelength range, normally incident light linearly polarized along afirst principal in-plane axis of the film body; and connecting the firstpacket of microlayers to the second packet of microlayers to form themultilayer optical film body, wherein at least some light can passthrough the first and second packets of microlayers sequentially;wherein the connecting is carried out such that: the first and secondpackets, in combination, have a first combined internal transmission ina range from 0.05 (5%) to 0.95 (95%) for the normally incident lightwhen averaged over the extended wavelength range; the first and secondpackets, in combination, have a second combined internal transmissionfor oblique light that is (a) incident at 60 degrees in a firstprincipal plane containing the first principal in-plane axis, and (b)linearly polarized in the first principal plane, the second combinedinternal transmission being in a range from 0.1 (10%) to 0.9 (90%) whenaveraged over the extended wavelength range; a reflection band of thefirst packet and a reflection band of the second packet overlap by atleast 70%; and at least in a first test area of the multilayer opticalfilm body, a combined high frequency spectral variability (Δcomb) of thefirst and second packets in combination is less than a first highfrequency spectral variability (Δ1) of the first packet by itself. 28.The method of claim 27, wherein the connecting is also carried out suchthat the combined high frequency spectral variability Δcomb is less thana second high frequency spectral variability (Δ2) of the second packetby itself.
 29. The method of claim 27, wherein the providing the firstand second packets and the connecting are accomplished by forming anextruded multilayer web and stretching the web to simultaneously formthe first and second packets of microlayers.
 30. The method of claim 27,wherein the providing the first and second packets is accomplished byforming a first multilayer optical film that includes the first packetof microlayers and separately forming a second multilayer optical filmthat includes the second packet of microlayers, and the connecting isaccomplished by laminating the first multilayer optical film to theseparate second multilayer optical film.
 31. The method of claim 27,wherein, at least in the first test area: the first packet ofmicrolayers exhibits a first transmission spectrum over the extendedwavelength range for the normally incident light, the first transmissionspectrum having the first high frequency spectral variability Δ1; thesecond packet of microlayers exhibits a second transmission spectrumover the extended wavelength range for the normally incident light, thesecond transmission spectrum having the second high frequency spectralvariability Δ2; and a difference between the first and secondtransmission spectra yields a first differential transmission spectrumover the extended wavelength range, the first differential transmissionspectrum having a first differential high frequency spectral variabilityΔdiff; and wherein the connecting is carried out such that the firstdifferential high frequency spectral variability Δdiff is greater thanat least one of the first high frequency spectral variability Δ1 and thesecond high frequency spectral variability Δ2.
 32. The method of claim31, wherein the connecting is carried out such that the firstdifferential high frequency spectral variability Δdiff is greater thaneach of the first high frequency spectral variability Δ1 and the secondhigh frequency spectral variability Δ2.